CHAP. XV Backboneless Animals 245 



and Paltidina ; the terrestrial snails and slugs are legion. Among 

 those of the shore the naked Nudibranchs are often in colour and 

 form protectively adapted to their surroundings ; those of the open 

 sea (Heteropods, Pteropods, and many cuttlefish) are active and 

 carnivorous, with light shells or none ; in the dark depths many 

 are blind or in other ways rudimentary, but food seems to be so 

 abundant that there is almost no need to struggle for it. 



As to diet, there are three kinds of eaters — carnivores, such as 

 the active swimmers we have mentioned, besides the whelks and 

 many other burglars who bore through their neighbours' shells, and 

 the Testacella slugs ; vegetarians, like the periwinkle, the snail, and 

 most slugs ; and thirdly, almost all the bivalves, which feed on 

 microscopic plants and animals, and on organic debris wafted to 

 the mouth by the lashing of the cilia on the gills and lips. In this 

 connection it is important to notice that all molluscs except bivalves 

 have in their mouths a rasping ribbon or toothed tongue {radula, 

 odontophore) by which they grate, file, or bore with marked effect. 

 Of parasites there are few, but one Gasteropod, Entocotuha 

 niirabilis, which lives inside the Holothurian Synapta, is very 

 remarkable in its degeneration. It starts in life as a vigorous 

 embryo like that of most marine snails, it becomes a mere sac of 

 reproductive organs and elements. 



In structure, molluscs differ remarkably from the arthropods 

 and higher "worms" in the absence of segments and serial 

 appendages. They are not divided into rings, and they have no 

 legs. 



To begin with, they were doubtless (bilaterally) symmetrical 

 animals, and this symmetry is retained in primitive forms like the 

 eight-shelled Chiton and in the bivalves. But most of the snails 

 are twisted and lop-sided, they cannot be symmetrically halved. 

 For this asymmetry the strange dorsal hump formed by the viscera, 

 and the tendency that the single shell would have to fall to one side, 

 are sometimes blamed. That this lop-sidedness is not necessarily 

 a defect, but rather the reverse, is evident from the success not 

 only of the snail tribe but of many other asymmetrical animals. 



The skin has a remarkable fold (double in the bivalves) known 

 as the " mantle," the importance of which in making the shell we 

 have already recognised. Another very characteristic structure is 

 the so-called "foot," a muscular protrusion of the ventral surface, 

 an organ used in creeping and swimming, leaping and boring, but 

 almost absent in the sedentary oysters. 



We rank the molluscs high among backboneless animals, partly 

 because of the nervous system, which here as elsewhere is a 

 dominating characteristic. There are fewer nerve centres than in 

 most Arthropods or in higher "worms," but this is in most cases 



