Facts

Squid are Teuthida, which comprises around 300 species. 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 strong swimmers and certain species can "fly" for short distances out of the water.

Overview
Introduction
Nervous System
Reproductive System
Digestive System

Modification from ancestral forms

Fossil Rhomboteuthis from the Lower Callovian of La Voulte-sur-Rhône, France

Squid have differentiated from their ancestral molluscs such that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. 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 ancestral shell has been lost, with only an internal gladius, or pen, remaining. The pen is a feather-shaped internal structure that supports the squid's mantle and serves as a site for muscle attachment. It is made of a chitin-like material.

Introduction

Composite diagram illustrating basic squid features (ventral aspect)

The main body mass is enclosed in the mantle, which has a swimming fin along each side. These fins, unlike in other marine organisms, are not the main source of locomotion in most species.
The skin is covered in chromatophores, which enable the squid to change color to suit its surroundings, making it practically invisible. The underside is also almost always lighter than the topside, to provide camouflage from both prey and predator.
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.^[3] In this form of locomotion, water is sucked into the mantle cavity and expelled out of the siphon in a fast, strong jet. The direction of the siphon can be changed, to suit the direction of travel.
Inside the mantle cavity, beyond the siphon, lies the visceral mass, which is covered by a thin, membranous epidermis. Under this are all the major internal organs.

 

Nervous system

The giant axon, which may be up to 1 mm (0.04 in) in diameter in some larger species, innervates the mantle and controls part of the jet propulsion system.
As cephalopods, squid exhibit relatively high intelligence among invertebrates. For example, groups of Humboldt squid hunt cooperatively, using active communication. (See Cephalopod intelligence.)

 

Reproductive system

A dissected male specimen of Onykia ingens, showing a non-erect penis (the white tubular structure located below most of the other organs)

A specimen of the same species exhibiting elongation of the penis to 67 cm in length

In females the ink sac is hidden from view by a pair of white nidamental glands, which lie anterior to the gills. There are also red-spotted accessory nidamental glands. Both organs are associated with food manufacture and shells for the eggs. Females also have a large translucent ovary, situated towards the posterior of the visceral mass.
Males do not possess these organs, but instead have a large testis in place of the ovary, and a spermatophoric gland and sac. In mature males, this sac may contain spermatophores, which are placed inside the female's mantle during mating.
Shallow water species of the continental shelf and epipelagic/mesopelagic zones are characterised by the presence of hectocotyli, specially modified arms used to fertilise the female's eggs.^[4] Most deep sea squid lack hectocotyli and have longer penises; Ancistrocheiridae and Cranchiinae are exceptions.^[4] Giant squid of the genus Architeuthis are unusual in that they possess both a large penis and modified arm tips, although it is uncertain whether the latter are used for spermatophore transfer.^[4] Penis elongation has been observed in the deep water species Onykia ingens; when erect, the penis may be as long as the mantle, head and arms combined.^[4][5] As such, deep water squid have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain sessile barnacles.^[4]

 

Digestive system

Like all cephalopods, squid have complex digestive systems. The muscular stomach is found roughly in the midpoint of the visceral mass. From there, the bolus moves into the caecum for digestion. The caecum, a long, white organ, is found next to the ovary or testis. In mature squid, more priority is given to reproduction such that the stomach and caecum often shrivel up during the later life stages. Finally, food goes to the liver (or digestive gland), found at the siphon end, for absorption. Solid waste is passed out of the rectum. Beside the rectum is the ink sac, which allows a squid to rapidly discharge black ink into the mantle cavity.

Ventral view of the viscera of the female Chtenopteryx sicula

Back to Top