Squidissection

Last Thursday, our class dissected squids. Squids are members of the phylum Mollusca along with snails, slug, clams, oysters, scallops and octopi. It may seem strange that all these different organisms were grouped together into one phylum but they share similar characteristics. Mollusks are soft-bodied organisms that have an internal or external shell. Bivalves are known for their external shells while squids possess an internal shell (the pen). There are a few types of mollusks that lack shells. Snails, slugs, clams and squids are also grouped into one phylum because they share the same basic body structure: foot, mantle, and visceral mass. They also share similar developmental patterns (most produce free-swimming larvae called trochophores).

The purpose of this dissection was to acquaint ourselves with the anatomy of a squid and to compare it with other mollusks. We noticed how the tentacles and arms were really just a modified version of a standard foot  and how the squid had an internal shell in comparison to clams and oysters who have external shells. Squids also possess radulas which are common to all mollusks except for bivalves. One glaring difference between squids and other mollusks was their well-developed compound eyes. I dissected one of the eyes and learned that the lens of the squid was actually quite large compared to its body (picture below).

 While squids are colour blind, they have highly developed sight and can somehow control their cromatophores to blend in with their environment. Unlike many sessile mollusks, squids are motile and agile. They use their funnels and jet propulsion to move themselves. This adaptation allows them to hunt effectively.

During the dissection, we were also able to find the ink sac. Many cephalopods squirt out a cloud of ink when they are threatened. This allows them to make a quick getaway. They squirt out ink from the funnel.
It was very interesting to try to write with squid ink. Unlike normal ink in our pens, it is not as fluid but quite dotty.

The squid we got had 8 arms and 2 tentacles. All of them have suckers to aid the squid catch prey.

The shorter arms grab and hold prey. They also bring it to the squid's mouth. The longer tentacles are for capturing prey that are further away. 

The squid moves by releasing water from its funnel (blue arrow) which propels itself in the opposite direction of where the funnel is facing.

The squid uses its arms and tentacles which are lined with suckers to capture and immobilize its prey. 

Another adaptation that helps the squid's predatory life are pigment cells called chromatophores. Chromatophores expand or contract to display or hide different colours which helps the squid surprise its prey or hide from predators. 

Squids, like many other mollusks, are bilaterally symmetrical and are coelomate. 

Another trait that squids have in common with other mollusks is the basic body structure. They all have a foot, a mantle that covers the body, and the visceral mass containing most of the organs. 

Squids have one pair of feathery gills for respiration (outlined in yellow).

The ink sac empties into the funnel. When the squid is agitated or feels threatened, the squid will shoot ink out of its sac and funnel, into the water to create a smokescreen of sorts to escape. 

The pen is the squid's internal shell. It supports the mantle, keeping the squid upright, allowing it to float in water. If the squid did not have a pen, squid's body would have no support and would just flop around. 

Solid waste exits the squid through the anus out of the siphon. Waste produced from cellular metabolism is removed from the blood by tube-like organs called nephridia. 

While it was difficult to distinguish the inner organs from one another at the beginning, everything became clear after we identified the ink sac. I had a very interesting time comparing the different structures of the squid to other mollusks. Its quite amazing to see how a basic body part could adapt into so many different forms depending on the purpose. The only thing that disappointed me in this dissection was the fact that we could not eat the squid afterwards. We threw out all the squid which could have been a delicious lunch.

On an end note, I think the squid would win: dinosaurs are extinct because they were unable to adapt to changes in the environment while squids are still around today. And the whale is too apathetic to get involved. Or maybe I'm just rooting for the squid because it is the cutest. 

Earthworm Dissection

After learning about Annelids in class, we all had a chance to dissect a earthworm. While it sounds like no big deal, I was pretty close to panicking; I am completely terrified of anything that creeps, crawls or slithers. To add to that, we were told that no gloves would be provided so we had to touch the worm with our hands. I almost died. Well, not really, but I wasn't too keen on the touching part, no matter how interested I was in the worm's anatomy. In the textbook, we saw many simplified diagrams of worms. So, naturally, this lab was aimed to let us find and observe these anatomies in a real specimen. It was a simple "look at the diagrams and identify the parts in the worm" kind of lab. However, this proved to be more challenging than I expected. I don't think any of us realized how simplified the diagrams were until we started dissecting the worm. The pictures in the textbook barely resembled the real anatomical structures. It was very tough to figure out what structure was what. In fact, some organs weren't even in the correct (standard) position due to our scalpels messing everything inside up. Also, the diagrams in the textbook were 2-D so we could see all the organs clearly. However, in the real specimen, the organs were layered one over one another and intertwined with others. This lab proved to me that while these worms may be considered primitive and are simple-looking, their appearances were deceiving as their anatomies were quite complex. It made me wonder just how complex and puzzling human bodies must be.

In this picture, we see the worm's five hearts which are also known as "aortic arches." They pump the blood through the worm's closed circulatory system. 

The digestive system starts at the mouth where the food enters. The pharynx pumps the food through the esophagus, through the crop (where it may be stored, to the gizzard (where the food is ground up). It then passes through the intestine and the waste is eliminated through the anus. 

Brain blocked by pin. Standard area of brain pointed out. 

The worm's "brain" is made up of paired ganglia (cluster of nerve cells) which are connected to the rest of the body by the ventral nerve cord. 

The nephridia deals with waste resulting from cellular metabolism. They remove waste products from body fluids and carry them to the outside. Solid waste is passed through the anus which is connected to the digestive system. 

We can assume the earthworms eat soil because of the large concentration of soil in the earthworm's digestive tract. If we wanted to test this theory out,  we can simply place a worm in an environment that only contains soil. If the worm survives, we can conclude that it eats soil. Or we can simply google what earthworms eat. 

Setae are tiny bristle-like hairs on the earthworm that help it anchor itself and move through the soil.

Photo Credit: http://images.tutorvista.com/content/animal-nutrition/digestive-tract-earthworm.jpeg

The small gizzard grinds up the relatively small amounts of food. The large amount of undigested ingested soil passes through the long intestinal tube. The difference in proportion between the gizzard and the intestine indicates the adaptation of the earthworm's digestive system.

While my dissection of the earthworm did not go past segment 32, I expect to see more of the intestine and the undigested food if I were to dissect the remainder of the worm/

Earthworms are hermaphroditic so they possess both male and female reproductive organs. The ovaries produce eggs and the testes produce sperm. However, worms do not self fertilize so it must join with another worm to reproduce. They connect at the clitellum and the sperm is released through the male genital pores, along the sperm grooves to the female genital pores to internally fertilize the eggs of the other worm. The clitellum then secretes a mucus ring into which the eggs and sperms are released. Then the ring slips off the worms and develops into a cocoon that shelters the fertilized eggs. 

While this lab was kinda icky in my opinion, I enjoyed the dissection immensely. I felt like a surgeon from a hospital drama. Through this lab, not only did we learn about the anatomical features of an earthworm, we also learned that creatures are more complex than textbooks make them out to be.