APPENDAGES OE MOLLUSCA. 325 



arched over by a fold of the mantle, occupies the hinder part of the 

 back, and so forms that region which is ordinarily known as the 

 ventral surface. To make these relations clear we must imagine 

 the animal placed in such a position that the aboral end points up- 

 wards, and the head forwards and downwards (cf. Fig. 172). All 

 the body above the head would then correspond to the dorsum of 

 the Gastropoda. The mantle is sometimes separated from the head 

 by a circular groove (Sepia) ; sometimes this fold of the mantle is 

 directly continuous with the integument of the head at the sides of 

 the neck (Octopus), so that the mantle forms a fold above the 

 branchial cavity only. Lateral processes of this mantle function as 

 locomotor organs (fins) ; in the Sepiada) they are generally small, 

 and extended along its whole length; in the Loliginidse they are 

 broader, but are limited to the aboral end of the body. 



The formation of the mantle-cavity and the position of the anus 

 lead us to the conclusion that this arrangement is due to the 

 primitive possession of a shell which covered the whole mantle ; and, 

 indeed, the shelled Cephalopoda are by far the older forms, while 

 the remarkable variations seen in the characters of their shells lead 

 us to think that this structure had a very ancient origin. 



An organ which has the same position as the foot of the 

 Gymnosomatous Pteropoda — the funnel — corresponds to the foot of 

 the Gastropoda. In Nautilus it is formed of two lamellae, which 

 arise from the ventral surface below the head, and which form a 

 tube by being rolled over one another ; this tube projects from the 

 mantle-cavity (Fig. 175, i). In the Dibranchiata this organ cannot 

 be seen to be composed of two lateral parts, except in the embryo; 

 they take their origin in the space between the mantle and the 

 rudiments of the arms. By growing together and gradually fusing 

 they form a tube which is similar to the one formed in Nautilus, 

 except that it is closed. The mantle, which is also muscular, is 

 attached to the periphery of the funnel; this effects powerful con- 

 tractions, and so drives out the water which has entered the 

 mantle-cavity between the funnel and the edge of the mantle ; and 

 the animal is driven in an aboral direction by the force produced by 

 the expulsion of this stream. The organ retains, therefore, its 

 primitive locomotor function. 



Appendages. 



§ 253. 



In the Mollusca the development of a cephalic region is 

 closely connected with the differentiation of processes, which I 

 regard as appendages, inasmuch as they are homologous with the 

 antennas and tentacles of Arthropoda and Vermes, and when more 

 highly differentiated are able to undertake the duties of appendages. 



