Differences Between Squid and Earthworm | Experiment

• Amine A. Harb

Introduction

Invertebrates are classified in relation to their lack of backbone or spinal column. A few distinguishing characteristics of invertebrates include distinctive respiratory organs, segmentation, radial and bilateral symmetries, and body organization.

This experiment required us to dissect and examine an earthworm and a squid. The squid’s taxonomic classification derives from the Family Loligindae, Order Teuthoidea, Class Cephaopoda, Phylum Mollusca, and Kingdom Animalia. The earthworm’s taxonomic classification derives from the Species Terrestris, Genus Lumbricus, Family Lambricicdae, Order Haplotaxida, Class Clitellata, Phylum Annelida, and Kingdom Animalia.

The objective of this lab is to dissect and examine the squid and earthworm. During the dissection, we were to detect and recognize any and all major external and internal features and structures of the two species. Also during this lab we were to comprehend and practice fundamental methods of dissection procedures and terminology.

Dissection Methods

Squid

• We first located and placed the squid, dorsal side up, in our dissecting pan with the tentacles and arms facing towards us. We examined the squid and pinpointed the head, eyes, beak, arms, tentacles, mantle, and skin.
• Utilizing our hand lends, we examined the suckers on the tentacles and the spotted areas on the skin.
• We noted the differences of the suckers of the tentacles and arms and their locations.
• Afterwards, we took a closer look at the chromatophores and listed our reasoning of what they are and their use.
• After wearing our gloves, we began to locate the dark beaks in the center of the mouth by moving the arms and tentacles aside.
• We then opened and closed the beaks in order to get an understanding of how the ventral beak overlaps the dorsal beaks.
• With our forceps we began to remove the beaks and place them together on the side of the dissecting pan with the dark parts pointed away from one another.
• Afterwards we began to search for the radula inside the mouth and then removed it by making small incisions using our scapula. We noted the function of the radula and placed it in a petri dish and examined it under our magnifying glass.
• We then went back to the body and turned it over, ventral side up, and located the funnel. We noted the functions of the funnel and its flexibility.
• After examining the funnel, we reoriented the squid’s arms and tentacles away from us in order to sketch and label the Tentacles, Arms, Head, Eyes, Fins, Mantle, Funnel, Tail, Suckers, Beaks, and Mouth.
• We then began to remove the eyeball using our forceps and then dissected it in order to locate the lens, pupil, retina, and iris utilizing our magnifying glass.
• While the squid is still on its back, we used to forceps to lift up the notch in the mantle behind the funnel and parted the mantle from the internal organs.
• While firmly pinching the mantle flesh with our forceps, we began to cut along the ventral midline of the mantle from its opening to the tail. While doing so, we made sure to keep the scissors lifted in order to leave the internal organs intact.
• After dissecting, we searched for the reproductive organs of our squid and located a white, fluid filled sac in the posterior end of the mantle. This allows us to understand that our squid was in fact a male.
• We then began to search for the gills, which are attached to the sides of the mantle, and began to examine and note the hearts of the gills.
• After noting the hearts and gills, we then moved to locate the digestive tract of the squid. We first located the ink sac, which is composed of a dark silvery tube on the bottom of the liver. Afterwards we noted the stomach and the caecum, lying together as a white tube. Around the stomach were what looks like human intestines, which are the digestive ducts of the squid.
• We then cut open the stomach utilizing our scapula to find no contents.
• After identifying the internal organs of the squid, we began to remove the ink sac utilizing our forceps and scapula. We held the ink sac gently with our forceps, pulling upwards, and began to slice the connective membrane along its length. After removal, we placed the ink sac in along the dissecting pan for further use.
• In order to write using our ink sac, we first are required to remove the gladius. The gladius, a long feather-shaped structure, will act as a pen for the ink. The gladius is utilized as a mantle support and for organ attachment in the squid.
• To remove the gladius we first grasped the head and organs firmly and rotated them to the side with my left hand. Using my right hand, I began to firmly hold the mantle in place and pulled gladius out gently. In order to fully remove it, my partner using our scapula to cut away the connective tissues that hold the gladius in place.
• After removing the gladius, we cut open the ink sac using our forceps and pressed it against the bottom of our dissecting tray. We then dipped the end of the gladius into the ink and used the ink-side to write the first letter of our name onto our worksheet.
• After writing a letter, we then began to draw, label, and identify the functions of the squid’s internal parts.
• We then cleaned up our dissecting tray and tools and began to prepare the table for the earthworm dissection.

Earthworm

• To begin the earthworm dissection, we first observed the external anatomy of the earthworm and noted the amount of segments our specimen had. We then identified the anterior and posterior ends of the earth worm and pinned it down to onto our dissecting tray.
• Utilizing our magnifying glass, we took a closer look at the tiny bristles on the ventral surface of the earthworm. Afterwards we moved our attention to the nephridiopores, which are small excretory pores.
• We then placed the earthworm on its ventral side, which is noticeably more flattened, and using our scalpel we made a shallow incision from the clitellum towards the mouth.
• Using our forceps, we spread the incision open and pinned the body wall to our dissection tray using our dissecting pins.
• We began to identify the septa, thin walls between each segment, utilizing our magnifying glass.
• Beginning at the mouth, we began to identify and note the organs of the digestive system. We first located the pharynx and then the esophagus extending from the pharynx. We then located the crop and the gizzard, simply due to their swollen structures.
• After locating the crop and gizzard, we moved onto the dorsal blood vessel. We located the dorsal blood vessel by moving along the dorsal surface of the digestive tract. Along the dorsal blood vessel we located the five pairs of aortic arches, which circle the esophagus.
• Afterwards we identified the cerebral ganglia, found along the dorsal surface of the pharynx. Along the cerebral ganglia, a ventral nerve cord can be observed extending the length of the worm.
• We then moved along the earthworm to locate the nephridia, which are the excretory organs. The nephridia can be found in each segment along the earthworm, more readily seen utilizing a magnifying glass.
• We then moved to segments 9 to 13 in order to note both of the male and female sex organs. The ovaries can be found in segment 13 and the testes can be found in segments 9 to 12.
• After noting all the major external and internal features of the earthworm we began to clean our dissecting tools and area.

Dissection Results

Photograph 1 – Squid: External Anatomy

Photograph 2a – Squid: Internal Anatomy

Photograph 2b – Squid: Internal Anatomy (Detailed)

Photograph 3 – Earthworm: External/Internal Anatomy

Anatomy and Physiology Discussion

Squid

Squids are soft-bodied cephalopods that are located throughout all the oceans (Project Oceanography, 2001). The mantle of the squid is used to protect the squid’s internal organs and regulate the water flow throughout the body (Project Oceanography, 2001). Contractions of the mantle direct the oxygen-rich waters across the gills in order for the squid to breathe (Project Oceanography, 2001). Harder contractions are utilized to produce thrust and propel the squid throughout its environment (Project Oceanography, 2001). The funnel of the squid is located behind and below the head of the squid and is flexible enough to propel the squid in many directions. The squid also utilizes its fins in order to provide additional lift to finely adjust the position of movement (Hotchkiss, Wilkes, & Ridenour, 2004).

Cephalopods also have the largest brains of all invertebrates and the brain is dominated by the optic lobe (Project Oceanography, 2001). The optic lobe is required to obtain information of its surroundings from the eyes (Project Oceanography, 2001). Similar to the eyes of mammals, the squid also has a lens, retina, iris, and cornea (Project Oceanography, 2001). Located below the head are the mouth, beak, and radula of the squid. The radula is a tongue-like structure with a belt of teeth. Along the radula, mouth, and beak, are the eight arms and two tentacles. The tentacles are used in order to quickly catch their prey and then the arms are used to bring the prey towards the squid and maneuvered into the mouth. Food is then grasped in the beak of the squid and then transported into the throat by the radula. Digestion of food in squids begins in the stomach, with the caecum also preforming some digestions (Project Oceanography, 2001). The stomach then dumps the contents into the intestines and finally the contents are then moved into the rectum and through the anus (Hotchkiss, Wilkes, & Ridenour, 2004).

Squids have three hearts, one at the base of each gill. The gill hearts receive blood that is rich in oxygen from the blood vessels and then pump the blood back into the systemic heart which serves the internal organs and the mantle (Hotchkiss, Wilkes, & Ridenour, 2004).

The skin of the squid is lined with chromatophores which consist of sacks of pigment that can be used to hide or show a given color. These chromatophores are generally used to create patterns that form communication among other squid (Project Oceanography, 2001). For defense, squids tend to squirt ink out of their ink sacs when panicked. This ink also contains a chemical that acts as a warning sign for other squids in the area (Project Oceanography, 2001).

In male squids, sperm is produced in the wall of the testis and are expelled out of the spermatophore into the female mantle cavity or at the base of the buccal cavity (Arkhipkin, 1992). Females produce eggs in the ovary, kept in the proximal end of the oviduct (Arkhipkin, 1992). Eggs usually pass out of the oviduct in clusters and are coated in proteinaceous secretions to aid in fertilization (Arkhipkin, 1992).

Earthworm

Earthworms are invertebrates that belong to the phylum Annelida, class Oligochaeta (Martin, Black, & Hawthorne, 1999). One of the basic requirements of earthworms is that they are required to stay moist in soil to remain active (Martin, Black, & Hawthorne, 1999). Advantages of moist soil include the ability to exchange gasses through a water film on the integument and to allow easier burrowing and movement (Martin, Black, & Hawthorne, 1999). Earthworm use bristle like organs that help them cling on to the slippery surfaces of soils located on the segments of the body (Martin, Black, & Hawthorne, 1999). Each of the segments located on the earthworm serve different functions. Alongside the anterior end of the earthworm are located the mouth and the prostomium. The prostomium is a lobe which acts as a covering for the mouth and is also used to wedge cracks open in the soil to allow the earthworm to crawl (Martin, Black, & Hawthorne, 1999). In addition to the bristle like organs alongside the earthworm, a mucus is secreted via various skin glands to aid in movement throughout the soil (Martin, Black, & Hawthorne, 1999).

The digestive tract of the earthworm is optimized for burrowing and feeding activities (Martin, Black, & Hawthorne, 1999). The earthworm swallows the soil and muscles mix the swallowed materials and pass it into the digestive tract. Enzymes in the digestive tract are then mixed with the materials and release amino acids, fluids, sugars, and other smaller organic molecules (Martin, Black, & Hawthorne, 1999).

Earthworms are hermaphroditic, but they do not self-mate (Martin, Black, & Hawthorne, 1999). A mutual exchange of sperm by two earthworms is required in order to mate. The sperm and egg cells are mixed with nutritive fluid and then deposited in cocoons produced by the clitellum (Martin, Black, & Hawthorne, 1999).

Conclusion

After dissecting and thoroughly inspecting both organisms, the group gained the ability to learn from first hand experiences about the major external and internal features and organs. We also gained the ability to understand and complete basic dissection techniques and terms.

References

Arkhipkin, A. I. (1992). Reproductive System Structure, Development and. Atlanta: J. Northway.

Hotchkiss, N., Wilkes, D., & Ridenour, S. (2004). Squid Lab. Baltimore.

Martin, J. P., Black, J. H., & Hawthorne, R. M. (1999). Earthworm Biology and Production. Gainsville: University of Florida.

Project Oceanography. (2001, Spring). Retrieved from www.marine.usf.edu: http://www.marine.usf.edu/pjocean/packets/sp01/sp01u7p2.pdf