226 



METAZOAN PHYLA 



Dorsal 



Fin 



Eye 



renfacles 



Pen 



5i-omarh 

 Caecvm 



K/^antle 

 ca^i-l'y 



Ink sac 



—Anas 



Pharynx 



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 equilib- 

 rium, or statocysts, and one which is 

 probably an olfactory organ. The eyes 

 (Fig. 137) 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 like 



a smoke screen behind which the animal makes its escape from an enemy. 



Another type of cephalopod is the octopus, or devilfish (Fig. 138), 



which has no shell but possesses a large bulbous body. The mantle 



Flo. 136. — Common squid of the 

 Atlantic coast, Loligo pealei Lesueur. 

 From a preserved specimen. X M. 

 A, lateral view. B, diagram to 

 show position of certain internal 

 structures. The body is placed 

 so as to bring out the position of the 

 morphological axes; its normal posi- 

 tion in the water is at an angle of 

 90 degrees to this, with the anterior 

 surface above (Sec. 264). 



