262 THE STUDY OF ANIMAL LIFE CHAP. 



mirabilis, 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 germ- 

 cells. 



In structure, molluscs differ remarkably from the 

 arthropods and higher " worms ' in the absence of seg- 

 ments and serial appendages. 



To begin with, they were doubtless bilaterally sym- 

 metrical animals, and this symmetry is retained in primi- 

 tive forms like the eight-shelled Chiton and in the bivalves 

 and cuttlefishes. But most of the snails are twisted and 

 lop-sided, they cannot be symmetrically halved. 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 a sign of concentra- 

 tion. There is a (cerebral) pair with nerves which supply 

 the head region, another (pleural) pair with nerves to the 

 sides and viscera, a third (pedal) pair whose nerves govern 

 the foot, and often other accessory centres of which 

 the most important are visceral. In the somewhat 

 primitive eight-shelled Chiton and its neighbours, the 

 nervous system is the most readily harmonised with 

 that of other Invertebrates ; in bivalves the three pairs 

 of^centres arc far apart ; in most snails and in cuttlefish 

 the three are concentrated in the head-region. Some 

 of the forms with concentrated ganglia show signs of 

 cleverness and emotion. 



