116 



MORPHOLOGICAL DEVELOPMENT. 



disunited. No true Mollusc multiplies by gemmation, either 

 continuous or discontinuous; but the product of every ferti- 

 lized germ is a single individual. 



It is a significant fact that here, where for the first time 

 we have homogenesis holding throughout an entire sub-king- 

 dom, we have also throughout an entire sub-kingdom no case 

 in which the organism is divisible into two, three, or more, 

 like parts. There is neither any such clustering or branch- 

 ing as a ccelenterate or molluscoid animal usually displays; 

 nor is there any trace of that segmentation which charac- 

 terizes the Annulosa. Among these animals in which no 

 single egg produces several individuals, no individual is 

 separable into several homologous divisions. This connexion 

 will be seen to have a probable meaning, on remembering 

 that it is the converse of the connexion which obtains among 

 the Annulosa, considered as a group. 



A Mollusc, then, is an aggregate of the second order. Not 

 only in the adult animal is there no sign of a multiplicity of 

 like parts that have become obscured by integration ; but 

 there is no sign of such multiplicity in the embryo. And this 

 unity is just as conspicuous in the lowest Lamellibranch as in 

 the highest Cephalopod. 



It may be well to note, however, more especially because 

 it illustrates a danger of misinterpretation presently to be 

 guarded against, that there are certain Molluscs which simu- 

 late the segmented structure. Externally a Chiton, Fig. 188, 



appears to be made up of 

 divisions substantially like 

 those of the creature Fig. 

 189; and one who judged 

 only by externals, would say 

 that the creature Fig. 190 

 differs as much from the 

 creature Fig. 189, as this 

 does from the preceding one. But the truth is, that while 

 190 and 189 are closely-allied types, 189 differs from 188 



