THE PHYLUM MOLLUSCA 963 



agus and digestive glands into the stomach. In the cephalopods both of 

 these secrete enzymes and the absorption of food appears to be Hmited 

 largely to the caecum. Enzymes rapidly liquefy the meat that is eaten, 

 and it is only the liquid hydrolysate that passes into the caecum. 



The ink sac opens just behind the anus into the end of the in- 

 testine. The glandular lining of this sac secretes a black liquid that is 

 expelled when the squid is alarmed. The defensive action of this ink has 

 been much debated. It is commonly thought to act as a "smoke screen" 

 behind which the squid can swim rapidly away. It may also serve as a 

 distracting dark object that momentarily holds the attention of the 

 pursuer. The ink of deep sea squids is luminescent, producing a bright 

 splotch in the otherwise black water. The ink of the octopus is known 

 to have an additional function. MacGinitie has shown that if a pur- 

 suer swims into the ink its sense of smell is paralyzed for as much as 

 two hours. During that time it will continue to hunt for the octopus, 

 but even if it touches it the pursuer seldom recognizes that the octopus 

 is there. We do not know whether squid ink has a similar effect. 



A single pair of gills hangs in the lower part of the mantle cavity. 

 Associated with these are a pair of auricles, nephridia and a single ven- 

 tricle, as in most molluscs. The circulatory system is closed, however, 

 unlike that of other molluscs. Arteries end in networks of capillaries all 

 over the body that come together in veins leading back to the nephridia. 

 Furthermore, all of the blood passing through the nephridia goes on 

 through the gills. Between each nephridium and gill is an auxiliary 

 gill heart that pumps blood through the capillary network of the gill 

 to the auricle (Fig. 14.15 C). 



Most of the central nervous system is grouped into a large ring 

 around the esophagus. This structure, the fused brain and ganglia, is as 

 large as the brain of a fish of similar size. It is also encased in a kind 

 of "skull," formed by several cranial cartilages. Many nerves run from 

 this central mass to all parts of the body. The only large ganglia out- 

 side of this center are the star-shaped stellate ganglia on the inner side 

 of the mantle. 



The large lateral eyes appear during development as simple pits 

 that resemble the pin-hole eyes of the nautilus. Later, however, a lens, 

 iris, cornea and focusing mechanism develop, producing an eye re- 

 markably like that of the vertebrates (Fig. 14.16). The lens is supported 

 on a flexible membrane between the inner and outer chambers. Con- 

 traction of the muscles around the inner chamber squeezes it and forces 

 the lens outward for near vision. The squid eye is "direct" since light 

 reaches the retina without having to traverse nerves and cell bodies. The 

 retina is ectodermal in origin, and is reached from behind by nerves 

 from the optic ganglia, large lateral outgrowths from the brain. As 

 shown in the figure, the lens is composed of two pieces. A unique feature 

 of the squid eye is that these two pieces form at different times during 

 development. The inner half develops along with the retina, while the 

 outer half forms later along with the iris. 



The apex of the body is occupied by the gonad. In the female, eggs 

 are released into a part of the coelom surrounding the ovary and col- 

 lected in a ciliated funnel to be stored in the oviduct. This loops back 



