PHYLUM MOLLUSCA 



295 



pharynx (buccal mass), esophagus, salivary 

 glands, stomach, cecum, intestine, rectum, 

 liver, and a small "pancreas." There are two 

 powerful horny jaws in the phar)'nx, and a 

 radula is also present. Above the rectum is 

 the ink sac, with a duct which opens near 

 the anus; this is a protective adaptation. 

 When the squid is attacked, it emits a cloud 

 of inky fluid through the funnel to provide 

 what had been thought to be a "smoke 

 screen" for its escape. However, recent stud- 

 ies show that the squid does not eject a large 

 cloud cover of ink as has been supposed; 

 it discharges just enough ink to color a 

 volume of water its own size. An enemy in 

 pursuit often mistakes the ink for the squid; 

 meanwhile the squid escapes. 



The blood of the squid is contained in a 

 double, closed, vascular system. Two gills 



and two nephridia are present. The nervous 

 system consists of a number of ganglia, 

 mostly in the head. The sensory organs are 

 two very highly developed eyes, two stato- 

 cysts, and probably two olfactory organs. 

 The eyes are large, and, superficially, some- 

 what similar to those of vertebrates (Figs. 

 184 and 397). 



In squids the sexes are separate. 



Squids are especially famous for their color 

 changes. Pigment cells filled with blue, 

 purple, red, and yellow color are present in 

 the skin; and when these become larger or 

 smaller, the color changes rapidly as though 

 the animal were blushing. These chameleon- 

 like changes in color harmonize with the 

 color of the background, resulting in partial 

 concealment of the animal. 



Near the coast of Newfoundland, giant 



Cartilage 

 Retina 



Cornea 

 Ciliary muscle 



Optic ganglion 

 Cartilage 



Figure 184. Diagram of a section through the eye of a squid. This eye is constructed on the 

 same principle as a camera, which consists of a dark chamber to which hght is admitted only 

 through an opening (pupil) in the diaphragm (iris). Behind this opening is a lens which 

 focuses the hght on the retina. It can form real images, as does the eye of the vertebrate, but 

 differs from the vertebrate eye in that light is received directly on the visual part of the retina 

 without having to traverse nerves and cell bodies before it strikes the light receptors as in the 

 vertebrate eye. 



squids are occasionally encountered. These 

 may be 50 feet or more in total length, with 

 arms as large as a man's legs, and suckers as 

 big as teacups. Probably certain sea serpent 

 stories are founded on the sudden and unex- 

 pected appearance of these monsters. The 

 giant squid is the largest living invertebrate 

 animal known. 



The cuttlefish has a short oval body bor- 

 dered by fins that are usually united behind 

 and possesses arms, two of which are much 

 longer than the others. Its internal calcare- 

 ous shell is the cuttlebone used as a bill 

 sharpener for caged birds. Its ink has pro- 

 vided the sepia pigment used by artists for 

 hundreds of years. 



