by James Morris
Photographs by Christine Kim
Recently, my students and I dissected a snake. I never dissected a snake before, even though I teach a class called Comparative Vertebrate Anatomy.
What’s fascinating about this exercise is that snakes are vertebrates, so for the most part have all of the same organs and structures as we do (with the notable exception of arms and legs). At the same time, the shape of their body is so different from ours, so extreme from our point of view, that familiar organs and structures have been modified in unfamiliar and sometimes surprising ways. As a result, a snake dissection provides all kinds of lessons not just about anatomy and evolution, but also about how we learn something new.
From the outside, a snake is essentially, well, snake-like, with a long and slender body, as can be seen from this snake skin –
What does it look like on the inside? The heart, lungs, GI tract, liver, and kidneys – the very same organs that are in us – have to somehow package themselves inside this long, cylindrical body. How do they all fit?
When we opened the snake and looked inside, my students and I noticed that familiar organs are modified in several ways. Perhaps most obviously, they are much more elongated than they are in you and me. The liver is long and stretched out, like a football. And so is the heart, lung, and stomach. In fact, all of the internal organs in a way mirror what’s on the outside.
But that’s not the whole story. Some organs are much reduced in size or completely missing. Consider the lungs. Humans and most vertebrates have a right and left lung. By contrast, snakes have a right lung, but are missing the left lung. The left lung is virtually gone, with only a small remnant of it … left. This vestigial lung is reminiscent of other structures that were once useful but no longer are, such as our appendix, the pelvis of whales, or the wings of flightless birds – all giving us clear windows into the evolutionary past.
And snakes have one more trick up their sleeve. Humans – like most vertebrates – have two kidneys. Snakes, evidently, can’t get away with just one kidney. So, how do they fit them side-by-side? The answer is they don’t. Unlike us, where the right and left kidneys are more or less symmetrical, the kidneys of snakes are decidedly asymmetrical. They aren’t side-by-side, but instead staggered, with one closer to the back end than the other.
Even the unpaired organs, like the heart, stomach, liver, pancreas, and spleen, are arranged one after the next, in a long row, inside the snake body.
We next asked some questions – how do snakes breathe with their single lung? Missing is the diaphragm – the dome-shaped muscle we use to breathe. The diaphragm is unique to mammals, so isn’t present in snakes and other reptiles. The lung of snakes is honey-combed, or, as one of my students put it, looks “exactly like uncooked Ramen noodles.”
And then we wondered where their legs would be if snakes had legs. The ancestors of snakes had legs, but they lost them over evolutionary time, perhaps as an adaptation to burrowing underground. But it’s possible to figure out where the legs would be if they were still present. Consider the hind legs – where would they be? About halfway down, so a snake would have a long tail? Or toward the back end, so that a snake has a short tail? It turns out that, in spite of their tail-like appearance, a snake has a very short tail.
The backbone is one of the most obvious features of snakes. Our vertebrae (backbones) are specialized – we have neck (cervical) vertebrae that help us move our head; thoracic vertebrae that attach to ribs; lumbar vertebrae that support our trunk; sacral vertebrae that form part of our hip; and coccygeal vertebrae that are remnants of a tail, now lost in humans. Snakes have many more vertebrae than we do – 200-400 (depending on the species) compared to 33 in humans. And most of the vertebrae in snakes are attached to ribs.
We also made some mistakes. When we first cut into the body, we found blocks of tissue that were easily separated from one another. We thought, at first, that they might be blocks of muscle. A little more prodding revealed – to everyone’s surprise – tiny snakes! This species of snake gives birth to live young, like we do. The young are nourished by the yolk of eggs that are retained inside the body of the mother. However, instead of laying eggs, she gives birth to live young.
My students and I learned all kinds of things about the anatomy of snakes, as well as vertebrates in general. We poked, prodded, explored, made observations, asked questions, took some guesses, made mistakes, talked as a group, and, by the end, learned something we didn’t know before. There was a lot of trial and error, false starts, and even misconceptions. At times we circled back – revisiting what we knew or thought we knew about vertebrates to guide us in learning something new.
We applied what we learned in fish, amphibians, reptiles, and mammals to figure out how the typical body plan is modified in snakes. As a colleague of mine put it, “We knew enough to learn more.”
Taking a step back, what I noticed about dissecting the snake is that learning is not a linear process, and seldom takes a straight path.
© James Morris and Science Whys, 2019
Special thanks to these motivated and dedicated students, who suggested that we dissect a snake: Alina Shirley, Christine Kim, Rachel Gerber, Annie Tsai, Grace Barredo, Kayla Shepherd, Abigail O’Brien, Maya Fields, Devin Feigelson, and Moshe Levenson.