Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles.
Squid are members of the class Cephalopoda, subclass Coleoidea, order Teuthida, which has two major suborders, Myopsina and Oegopsina. Squid are strong swimmers and certain species can “fly” for short distances out of the water.
- What before may have been the foot of the ancestor is modified into a complex set of tentacles and highly developed sense organs, including advanced eyes similar to those of vertebrates.
- The pen is a feather-shaped internal structure that supports the squid’s mantle and serves as a site for muscle attachment.
- The skin is covered in chromatophores, which enable the squid to change color to suit its surroundings, making it practically invisible.
- Under the body are openings to the mantle cavity, which contains the gills (ctenidia) and openings to the excretory and reproductive systems.
- At the front of the mantle cavity lies the siphon, which the squid uses for locomotion via precise jet propulsion.
- In this form of locomotion, water is sucked into the mantle cavity and expelled out of the siphon in a fast, strong jet.
- Inside the mantle cavity, beyond the siphon, lies the visceral mass, which is covered by a thin, membranous epidermis.
- The giant axon, which may be up to 0.04 in in diameter in some larger species, innervates the mantle and controls part of the jet propulsion system.
- In females, the ink sac is hidden from view by a pair of white nidamental glands, which lie anterior to the gills.
- Males do not possess these organs, but instead have a large testis in place of the ovary, and a spermatophoric gland and sac.
- Giant squid of the genus Architeuthis are unusual in that they possess both a large penis and modified arm tips, although whether the latter are used for spermatophore transfer is uncertain.
- Two branchial hearts feed the gills, each surrounding the larger systemic heart that pumps blood around the body.