MOLLUSCA— OUTER ORGANISATION 



37 



with the exterior at the free edge of the mantle fold, above the 

 "head." Within the mantle cavity there are two or four gills, 

 arranged symmetrically, the median anus, and the apertures of the 

 sexual and excretory organs. Two symmetrical lobes are found on 

 the posterior lower side of the visceral dome ; the edges of these 

 are apposed in such a way as to form a tube, the funnel or siphon, 

 one aperture of which lies in the mantle cavity, Avhile the other 

 protrudes from the mantle cleft. The respiratory water enters the 

 mantle cavity through the mantle cleft, and escapes through the 

 siphon. The faecal masses, waste and sexual products, and the 

 secretion of the ink-bag also leave the body through the siphon. 

 Originally, no doubt, all Cephalopoda possessed a shell which covered 

 the Avhole visceral dome as Avell as the mantle fold. In recent 

 Cephalopods the shell is rarely developed in this way ; it is often 

 rudimentary, and may, indeed, be altogether wanting. Eecent 

 Cephalopods fall into two entirely distinct divisions, the Tetra- 

 branchia and the Dibranchia. 



The Tetrabranehia (Nautilus, Fig. 48). 



These have a shell coiled anteriorly (exogastrically) in the plane 

 of symmetry, and divided by septa into consecutive chambers. 

 The animal occupies 



the last and largest ?° 



chamber ; the others 

 contain gas. ^ The septa 

 separating the consec- 

 utive chambers are 

 pierced in the middle 

 to allow of the passage 

 of a siphunele, which 

 runs through all the 

 compartments, and is 

 attached to the visceral 

 dome. That portion of 

 the foot which sur- 

 rounds the mouth is 



produced intonumerOUS ^^^^ 4S.-Diagram of Nautilus, left view, ve, ventral ; do, 



tentacles, which can be dorsal; ro, anterior; /it, posterior;/, foot (tentacles and .siphon); 



retracted into special *'™' shell muscle ; ct, ctenidia ; mh, mantle cavity ; a, anus ; s, 



•1 , 1 shell ; si, siplmncle ; ou, eye ; o, mouth. 



The anterior poi^tion of the foot, which lies in front of and over 

 the head, is widened out into a concave lobe, the hood ; this is applied 

 to the outer surface of the occupied chamber of the shell anteriorl)^, 

 and, when the tentacles are withdrawn, can close its aperture. The 

 hood carries two tentacles, and on each side of the head there is an eye. 



^ Or water ; r. Ford's Introduction to Brit. Mus. Cat., Fossil Cephalopoda, 1889. 



