Squids

Squids 🦑 are cephalopods with elongated bodies, large eyes, eight arms and two tentacles, in the two orders Myopsida and Oegopsida. 

These were formerly regarded as two suborders of the order Teuthida, but Teuthida is now thought to be paraphyletic. The two current orders comprise around 304 species.

1920px-Loligo_vulgaris

European squid (Loligo vulgaris). image: Hans Hillewaert/wikipedia

 Like all other cephalopods, squids have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.

Squid can change colour for camouflage and signalling. Some species are bioluminescent, using their light for counter-illumination camouflage.


Description

The head and foot of the squid are at one end of a long body, and this end is functionally anterior, leading the animal as it moves through the water. The foot has been transformed into a set of tentacles, or arms, which surround the mouth. These are flexible and prehensile, and usually bear disc-like suckers. A ventral part of the foot has been converted into a funnel through which water exits the mantle cavity. 

Composite_diagram_illustrating_basic_squid_features,_ventral_aspect

Composite diagram illustrating basic squid features (ventral aspect). image:  NOAA Technical Report NMFS 73/ wikipedia


The main body mass is enclosed in the mantle, which has a swimming fin along each side. These fins are not the main source of locomotion in most species. The mantle wall is heavily muscled and inside, the visceral mass, which is covered by a thin, membranous epidermis, forms a cone-shaped posterior region known as the "visceral hump". The mollusc shell is reduced to an internal, longitudinal chitinous "pen" in the functionally dorsal part of the animal; the pen acts to stiffen the squid and provides attachments for muscles.

On the functionally ventral part of the body is an opening to the mantle cavity, which contains the gills (ctenidia) and openings from the excretory, digestive and reproductive systems. 

How they move

Squids have an inhalant siphon behind the funnel draws water into the mantel cavity via a valve. The squid uses the funnel for locomotion via precise jet propulsion. In this form of locomotion, water is sucked into the mantle cavity and expelled out of the funnel in a fast, strong jet. 

The direction of travel is varied by the orientation of the funnel.  Squid are strong swimmers and certain species can "fly" for short distances out of the water.

Commentary

Curiously the Apple emoji of the squid could be slightly incorrect 🦑

enlarged image of squid emoji from Apple.


The funnel (siphon) hole should be on the other side (dorsal side), although it is easy to see how this error could be made based on the standard diagram is ventral. Luckily it is pretty small most of the time. ref


Source adapted from: Wikipedia contributors. (2018, December 8). Squid. In Wikipedia, The Free Encyclopedia. Retrieved 03:06, December 9, 2018, from https://en.wikipedia.org/w/index.php?title=Squid&oldid=872720263