THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 103 



phases of morphological composition up to the highest forma 

 in any sub-kingdom, we find ourselves at the extremity of a 

 great branch, from which there is no access to another great 

 branch, except by going back to some place of bifurcation low 

 down in the tree. 



The nearest relatives of the MoUusca are those molluscoid 

 forms treated of early in the last Chapter. A Brachiopod or 

 a solitary Ascidian, though widely unlike a Mussel or a 

 Snail or a Cuttle-fish, is nearer akin to them than is any 

 coelenterate animal or annulose animal or vertebrate animal. 

 One of the leading distinctions, however, between the Mol- 

 luscoida and the Mollusca, considered as groups, is that 

 \vhereas the Molluscoida are very frequently, or indeed 

 generally, compound, the Mollusca are invariably single. 

 No true Mollusk multiplies by gemmation, either continuous 

 or discontinuous ; but the product of every fertilized 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- 

 kingdom, we have also throughout an entire sub-king- 

 dom no case in which the organism is divisible into two, 

 three, or more, like parts. There is neither any such 

 clustering or branching as a coalenterate or molluscoid ani- 

 mal usually displays ; nor is there any trace of that seg- 

 mentation which characterizes 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 Mollusk, 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 Lamelli- 

 branch as in the highest Cephalopod. 



