. Animal biology. Zoology; Biology. 222 METAZOAN PHYLA Dorsa/ Pen 5tomarh Caecum h/lantle cavi-f-y Ink sac Anas Pharynx Mou-f-h In the case of both the squid and cuttlefish, the mantle incloses a cavity on the lower surface and ends just behind the head in a free margin, or collar. From under this collar projects a siphon, out of which water can be forced by the contraction of the mantle, driving the animal back- ward through the water. The shell in the cuttlefish is a horny and limy plate con- cealed under the skin of the upper surface of the body; it is formed by a pocket of the mantle. In t
. Animal biology. Zoology; Biology. 222 METAZOAN PHYLA Dorsa/ Pen 5tomarh Caecum h/lantle cavi-f-y Ink sac Anas Pharynx Mou-f-h In the case of both the squid and cuttlefish, the mantle incloses a cavity on the lower surface and ends just behind the head in a free margin, or collar. From under this collar projects a siphon, out of which water can be forced by the contraction of the mantle, driving the animal back- ward through the water. The shell in the cuttlefish is a horny and limy plate con- cealed under the skin of the upper surface of the body; it is formed by a pocket of the mantle. In the squid it is only horny. There are two fins, one on each side of the body toward the tail end, which may be used for forward locomo- tion and as a means of directing the course of the animal. When swimming forward the tentacles are pressed together, are extended in front, and are used for steering. The siphon may also determine the direction of backward motion by being pointed either toward or away from the head as the water is forced through it. There are two power- ful chitinous jaws in the squid resem- bling an inverted parrot's beak, and a radula is present. There are also two gills in the mantle cavity. The nervous system consists of sev- eral pairs of ganglia brought together in close proximity in the head. The sen- sory organs include two very highly developed eyes, two organs of equihb- rium, or statocysts, and one which is probably an olfactory organ. The eyes (Fig. 128) are large and superficially similar to those of the vertebrates but when critically compared with them are found to be fundamentally different in structure. The resemblance is a case of analogy. The eyes of cephalopods are capable of distinct vision—that is, they have the power to form a picture. Squids possess a gland known as the ink sac that secretes a black coloring matter which, when spread in the water, produces a cloud hke a smoke screen behind which the animal makes its escape from an enemy
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