316 



INVERTEBRA TE MORPHOLOG T. 



usually brilliant coloration. The foot in the pelagic Pliyl- 

 lirhoe has entirely disappeared, but is usually elongated and 

 provided with a broad flat surface, in accordance with the 

 creeping habits of the Nudibranchs. Parapodial folds, such 

 as occur in the Tectibranchs, are never developed. 



Order Pulmonata. 



The Pulmouates differ from all the other groups of Gas- 

 ^teropods in that they are, with the exception of a single genus, 

 Onchidium, either terrestrial or aquatic ; and in adaptation to 

 this assumed habit certain well-defined changes have occurred. 

 In some genera, more especially the aquatic forms, such as 

 Limncea, Physa, and Planorbis, the visceral hump has its typi- 

 cal Gasteropod development, and is spirally coiled ; but in 

 many terrestrial forms, such as Limax (Fig. 142, A], Arion, 



and Fagimda, it is low and elon- 

 gated parallel to the long axis 

 of the foot with which it is 

 fused. The mantle is in all 

 forms well developed, but pre- 

 sents the peculiarity that it is 

 fused by its edges to the body- 

 wall except at one point upon 

 the right side, where an open- 

 ing is left by which the other- 

 wise completely-closed mautle- 

 ' cavity communicates with the 

 exterior and through which air 



may be taken into the cavity. The position of the mantle- 

 cavity, when not interfered with by secondary changes, is 

 upon the right side of the body and somewhat in front of 

 the visceral hump when this is present. A spirally-coiled 

 shell is present in all forms in which the visceral hump 

 is well developed, as in Limncva, Physa, Helix (Fig. 142, 13], 

 and Planorbis, but in the elongated terrestrial forms a 

 rudimentatiou of the shell accompanies the diminution of 

 the visceral hump. Thus in Daudebardia, in which only 

 a slight trace of the hump persists, the shell has become 



FIG. 142. A, Limax maximum, 

 Helix (after HOWES). 



