CEPHALOPODA 603 



transverse partitions. The shell of Argonauta, the paper nautilus, is not 

 homologous to that of other cephalopods. In the Decapoda, the shell 

 is concealed in a pocket of the anterior wall of the mantle. It differs 

 much in shape and composition in the various species, being calcareous in 

 Spirilla and Sepia, but coiled in the former and straight in the latter, and 

 ehitinous and elongate in the squids. 



The foot is absent in cephalopods in the form in which it occurs in 

 other mollusks. The siphon, however, represents the fused epipodia and 

 the ai-ms are lateral processes of the foot, the basal portion of which has 

 grown for\v^ards and enveloped the head, so that the mouth comes to lie 

 in their midst. 



Internal Structure. —The mouth is surrounded by a ring-shaped lip 

 and leads into a muscular phaiynx, A pair of sharp ehitinous or partly 

 calcareous jaws, which look a good deal like a parrot^s beak, lie just back 

 of the lip, by means of which the animal tears its prey to pieces. In the 

 phai-ynx are the tongue and the radula. The radula is small, as com- 

 pared with the size of the animal, and consists of a central tooth and 

 three laterals in each transverse row; marginals are absent in the 

 Decapoda, and are represented in the Octopoda by a plate of varying 

 shape. Two pairs of salivary glands pour their secretion into the 

 pharynx; in the Decapoda the anterior glands are united or may be 

 rudimentary. 



The oesophagus is a narrow tube which leads to the stomach; in 

 Nautilus and the Octopoda a lateral crop is present. The stomach is a 

 muscular organ with a large thin-walled caecum, which acts as a gall 

 bladder and receives the secretion of the large liver. The two ducts which 

 pass to the stomach caecum from the liver have in their walls a paired 

 gland called the pancreas. The intestine leaves the stomach and passes 

 forward to the anus in the mantle cavity. A glandular pocket at the 

 anal end of the rectum, called the ink sac, secretes a brown or black 

 fluid, by means of which the animal clouds the water when fleeing from 

 an enemy ; Nautilus is without the ink sac. 



The respiratory organs consist of a" pair of ctenidia (two pairs in 

 Nautilus) which lie in the mantle cavity. These bear a close relation to 

 the circulatory system. There are three hearts. The systemic or arterial 

 heart consists of two auricles (four in Nautilus) which receive blood from 

 the gills and pour it into a median ventricle, which distributes it through 

 arteries throughout the body. The blood returns through large veins to 

 the two branchial hearts at the base of the gills, which send it through 

 these organs. 



The excretory organs consist of a pair of kidneys (two pair in 

 Nautilus), which open into the mantle cavity near the anus; they also 



