MOLLUSC A. 449 



normally along the ventral surface below and behind the head to the 

 posterior limit of the body. It varies in size and shape, and may be 

 simple (Lamellibranchiata, many Gastropoda), or divided transversely into 

 segments, the pro-, meso-, and meta-podium (some Gastropoda}. In the 

 Pteropoda the median part of the foot is rudimentary : but there are two 

 large lateral swimming lobes, homologous probably with the epipodial 

 lobes of the Gastropoda, and, like them, supplied by nerves from the pedal 

 ganglion. The foot is still more modified in Cephalopoda. One portion 

 of it, the fore-foot, as proved by the nerve-supply from the pedal ganglia, 

 has grown round the head, and is produced into lobes or arms character- 

 istic of the class. It may correspond with the lateral lobes of the foot in 

 Pteropoda which grow round the mouth in the order Thecosomata. Another 

 portion of the foot the mid-foot forms two lobes which usually fuse 

 together, and constitute the siphon : and a third portion the hind-foot 

 is represented by the valve of the siphon. The homologies of these three 

 divisions of the foot are by no means certain, as the development of this 

 region of the body is apparently greatly altered by the presence in it 

 of a plentiful food-yolk. 



The shell is essentially a cuticular structure, calcified by Lime 

 carbonate with the exception of a thin superficial layer, the periostracum or 

 epicuticula ; but occasionally calcification fails to take place. In Di- 

 branchiate Cephalopoda the shell is developed when present in a sac formed 

 by the closure of two dorsal integumentary folds. In Spirula alone do 

 these folds fail to unite. The way in which the shell is first formed in 

 Tetrabranchiate Cephalopoda is unknown. In all other Mollusca the larva 

 possesses a shell-gland developed as a pit of the epiblast on the dorsal 

 aspect of the visceral dome. This gland rarely persists in the form of 

 a sac, as in a few Pulmonate Gastropoda, but it is usually everted, and on 

 this everted surface the shell commences its development. Its subsequent 

 growth is dependent on the mantle. It extends with the extension of the 

 mantle-edge, and an increase of thickness takes place from the surface 

 of the mantle already covered. In some instances, e.g. Tetrabranchiate 

 Cephalopoda and a few Gastropoda, the animal as it grows in size, 

 quits a portion of the shell which is then closed off by a calcareous septum, 

 The processes and ridges of the shell are formed by folds and processes 

 of the mantle-edge, its colours by pigment-secreting glands in the same 

 region. The actual shape of the shell is very variable. It is composed of a 

 single piece, except in the Gastropoda Isopleura where it consists of a longi- 

 tudinal series of plates one behind the other, and the Lamellibranchiata 

 where it consists of a right and left valve, connected, however, by a median 

 dorsal ligament to be regarded as an uncalcified portion of the shell. 



The dorsal or antipodial region of the body has a thin integument, 

 and constitutes the visceral dome or hump. It is scarcely marked in 



