Snout (rostrum)The pointed front of the head. Its skin is dotted with pores that lead to jelly-filled sense organs, the ampullae of Lorenzini.
NostrilOne of a pair on the underside of the snout. A flap of skin divides each one, so water flows through a sac for smelling.
EyeAn oval eye on each side of the head. Its upper and lower lids do not move.
SpiracleA small round opening just behind the eye. It is the first gill slit, much reduced, and it opens into the throat.
MouthA curved slit on the underside of the head, not its tip. Both jaws carry rows of teeth.
Gill slitsFive long slits on each side. Water taken in at the mouth passes over the gills and leaves here. A shark has no gill cover.
Pectoral finOne of the front pair of fins, just behind the gill slits. They are stiff and give lift and steering.
First dorsal finThe front fin on the back. In the spiny dogfish a spine stands at its front edge.
Fin spineA sharp spine stands in front of each dorsal fin. It gives the spiny dogfish its name. The spines are for defense.
Second dorsal finThe rear fin on the back, with its own spine in front.
Pelvic finOne of the rear pair of fins. It sits under the body where the trunk meets the tail. The cloaca opens between the two.
Caudal fin (tail)The backbone bends up into the longer upper lobe. A tail of this kind is called heterocercal.
Lateral lineA line of sense organs along each side. It detects movement and vibration in the water.
The organs 8
LiverA very large, oily organ in long lobes. Its oil helps the shark stay up in the water, and the liver also makes bile.
StomachA large J-shaped sac. Its wide straight part runs back, then it bends and narrows to its exit, the pylorus.
SpleenAn organ on the outside of the bend of the stomach. It belongs to the lymphatic system, not the gut.
DuodenumThe short first part of the intestine, just after the pylorus. The bile duct from the gallbladder opens into it.
PancreasA gland lying in the curve of the duodenum. It makes digestive enzymes that pass into the intestine.
Spiral valveA spiral fold of lining inside the wide intestine. It gives a short gut a large surface for taking in food. The rings outside show where the fold is attached.
Rectal glandA small finger-shaped gland. A duct joins it to the end of the intestine. It marks where the very short large intestine begins.
CloacaThe end chamber, opening between the pelvic fins. The intestine and the ducts of the kidneys empty into it.
The heart 4
Sinus venosusThe first chamber: a thin-walled sac at the back of the heart. It collects the blood from the veins.
AtriumThe second chamber: a large thin-walled sac. It spreads out on both sides of the ventricle. Also called the auricle.
VentricleThe third chamber: thick-walled and muscular, with its point toward the tail. It pumps the blood onward.
Conus arteriosusThe fourth chamber: a thick-walled tube that runs forward from the ventricle. It sends the blood on to the ventral aorta and the gills.
How this list was compiled: Every name checked on 2026-09-30 in two sources. L. H. Hyman, A Laboratory Manual for Comparative Vertebrate Anatomy (1922, public domain), the dogfish: its outside, the organs of the body cavity and the four chambers of the heart, https://archive.org/details/ost-biology-labmanualcompvert00hymarich. R. W. Hegner, College Zoology (1912, public domain), the dogfish shark: fins and spines, spiracle and gill slits, lateral line, stomach, spiral valve, liver, pancreas, spleen and heart, https://archive.org/details/collegezoology00hegn. Wikipedia, Shark anatomy, Spiny dogfish, Lateral line, Spiral valve and Fish anatomy, for the fins, the missing anal fin, the rectal gland, the cloaca and the heart, https://en.wikipedia.org/wiki/Shark_anatomy. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work
How it works
A shark diagram labeled in three views, drawn on the spiny dogfish, the small shark many teaching labs use: the outside from the left, the organs of the body cavity, and the heart. It is a plan, not to scale.
Read the full guide
Outside
Start at the snout and work back: nostril, eye, spiracle, mouth and the five gill slits, then the fins. The spiny dogfish has a spine in front of each dorsal fin and no anal fin. The dashed line along its side is the lateral line.
Organs and heart
Switch to Organs for the shark digestive system diagram: the J-shaped stomach, the short duodenum with the pancreas in its curve, and the wide intestine with the spiral valve inside. Heart is the shark heart diagram: chambers in a row, with arrows for the blood going forward to the gills. Press Quiz me to name each part yourself.
A shark's mouth and nostrils are on the underside of its head. Look below the snout, not at its tip.
Questions and answers
How many fins does a shark have?
Most sharks have eight: two pectoral fins, two dorsal fins, two pelvic fins, an anal fin and the caudal fin. The spiny dogfish drawn here has no anal fin.
What is the spiral valve in a shark?
A spiral fold inside the wide part of the intestine. A shark's intestine is short, and the fold gives it a large surface for taking in food and slows the food down.
Why is a shark's liver so big?
It is full of oil. A shark has no swim bladder, and the oil makes its body less dense, which helps keep it from sinking. The liver is also the shark's energy store.
How many chambers does a shark's heart have?
A dissection manual counts four in a row: sinus venosus, atrium, ventricle and conus arteriosus. Other books count only the atrium and the ventricle and say two. The blood goes one way, forward to the gills.