MOLLUSCS. 



363 



Fig. ili.— Argonauta with shell broten to show the eggs. 



space will at present allow us to allude, that of the paper sailor, Argonauta. In this 

 genus the shell is without connection -with the body, but is secreted by two of the 

 arms, which are greatly expanded for 

 the purpose, and serves as a nest for the 

 eggs. 



The mouth of the cephalopods is 

 armed with a pair of beaks like those of 

 a parrot, with sharp cutting edges. In 

 the Nautilus these beaks are calcified, 

 but in all other living forms they are 

 horny. Between the jaws is the tongue, 

 and below them is a lingual ribbon, to 

 all intents and purposes like that of the 

 cephalophorous MoUusca. Behind the 

 lingual ribbon begins the usually narrow oesophagus, into which one or two pairs of 

 salivary glands pour their secretions. The oesophagus empties into the stomach, 

 which is of moderate size, but strong and muscular, and provided with a blind appen- 

 dage (csecum), which frequently is larger than the stomach itself. From the stomach 

 the intestine goes forward in a course nearly parallel to that followed by the oesopha- 

 gus, and terminates in a median vent on the lower surface of the body just within the 

 siphon. The liver is large, and opens into the osecum. In the dibranchiate forms 

 there is an ink-bag, borne near the intestine, and poui-ing its inky secretions into the 

 rectum, or into the mantle cavity through an opening near the anus. The ink is brown- 

 ish or black, and is used to create a cloud in the water when the animal wishes to 

 escape from some danger. The ink is not readily decomposed ; on the contrary, it is 

 occasionally found fossil in the rocks along with the remains of the animal which pro- 

 duced it. So well has it been preserved that in one 

 celebrated instance a naturalist drew the portrait of a 

 fossil squid with the sepia derived from its fossil, but 

 not fossilized, ink-bag. 



The circulatory system is well developed in all the 

 cephalopods, especially in the higher groups. The de- 

 tails would prove dry reading, and so we will merely 

 mention a few facts. There is only one ventricle, but 

 the auricles may vary from two to four, in accordance 

 with the number of gills. On each vein going to the 

 gills is a pulsating vesicle, the branchial heart. It is 

 in the problematical aj)pendages to these that the 

 curious worms, Dicyemida, referred to on a preceding 

 page, have been found. In some true capillaries are 

 Wv" developed, while in others the arteries terminate in the 

 lacunsE of the body. We have just referred to the ex- 

 istence of one or two pairs of gills. This character is 



riG.475.yXervoussy^stem^o«em associated with Others, and is used in the naming and 



tikis ; 



ITv&^^'^-l^f'^^l^'^^X^^ definition of the two primary groups, Dibranchiata and 

 s, optic nerve. Tetrabranohiata. Correllated with this variation in the 



number of gills and hearts is one in the kidneys, which are also two or four in number. 



It is in these points only that any metamerism of the cephalopods is noticeable. 



