432 MANUAL OF ZOOLOGY. 



cups or suckers. Four of the arms of the male are specially 

 modified to form a peculiar organ termed the " spadix," which 

 is connected with reproduction, and corresponds with the 

 " hectocotylised " arm of the male Cuttle-fishes. In the centre 

 of the head is the mouth, surrounded by a circular fleshy lip, 

 external to which is a series of labial processes. The mouth 

 opens into a buccal cavity, armed with two horny mandibles, 

 partially calcified towards their extremities, and shaped like 

 the beak of a parrot, except that the under mandible is the 

 longest. There is also a " tongue," which is fleshy and sen- 

 tient in front, but is armed with recurved teeth behind. The 

 gullet opens into a large crop, which in turn conducts to a 

 gizzard, and the intestine terminates at the base of the fun- 

 nel. On each side of the crop is a well-developed liver. 



The heart is contained in a large cavity, divided into sev- 

 eral chambers, and termed the "pericardium" (Owen). The 

 respiratory organs are in the form of four pyramidal branchiae, 

 two on each side. 



The chief masses of the nervous system are the cerebral 

 and infra-cesophageal ganglia, which are partially protected by 

 a cartilaginous plate, which is to be regarded as a rudimentary 

 cranium, and which sends out processes for the attachment of 

 muscles. The organs of sense are two large eyes, attached 

 by short stalks to the sides of the head, two spheroidal ear- 

 capsules, and two hollow plicated subocular processes, be- 

 lieved to be possibly olfactory in their function. 



The reproductive organs of the female consist of an ovary, 

 oviduct, and accessory nidamental gland. 



There is no ink-bag, and the funnel does not form a com- 

 plete tube, but consists of two muscular lobes, which are 

 simply in apposition. It is the organ by which swimming is 

 effected, the animal being propelled through the water by 

 means of the reaction produced by the successive jets emitted 

 from the funnel. The function of the chambers of the shell 

 appears to be that of reducing the specific gravity of the 

 animal to near that of the surrounding water, since they are 

 most probably filled with some gas secreted by the animal. 

 Good authorities, however, believe that the chambers of the 

 shell are filled with water. The function of the siphuncle is 

 unknown, except in so far as it doubtless serves to maintain 

 the vitality of the shell. 



SHELL OF THE TETRABRANCHIATA. The shells of all the 

 Tetrabranchiata agree in the following points : 



1. The shell is external. 



2. The shell is divided into a series of chambers by plates 



