144 THE INVERTEBRATA 



sense organs, of the "brain", and usually also of the mouth, and is 

 often obviously differentiated as a head. In a segmented animal this 

 cephalization may extend to one or more of the anterior somites ; and 

 these usually become part of the head, losing their individuality in 

 the way mentioned in the preceding paragraph, and only betraying 

 their existence by the presence of certain of their organs (ganglia, 

 appendages, etc.). 



In the process of development by which the body peculiar to the 

 species is reconstituted from the ovum, the early stages are of 

 necessity much unlike the adult; but because the general features 

 must arise before the more special ones, and because general features 

 are shared by animals, the young resembles other young animals 

 which have reached the same stage. Since the more special a feature 

 is the fewer are the animals which share it, as the young approaches 

 the adult form the circle of other animals whose young it resembles 

 narrows. Since the evolution of its species consisted in the appear- 

 ance of the same special features, its development (ontogeny) roughly 

 recapitulates its evolution (phylogeny), but its features at any moment 

 are not those of the adult of some ancestor but those of the corre- 

 sponding young stage of that ancestor, and it is only because that 

 stage was preparing the features of its own adult that there is re- 

 capitulation of the latter. Not all features of young animals, however, 

 are anticipatory of those of their adults. Some of them — the em- 

 bryonic membranes of the higher vertebrates and the ciliated bands 

 of echinoderm larvae for instance — are adaptations to the needs of 

 the young only and disappear in the adult. Such features are said to 

 be caenogenetic . In respect of them development in no sense re- 

 capitulates adult phylogeny. Now it is held that in some cases a 

 young animal, becoming sexually mature at an early stage (as in the 

 well-known instance of the axolotl which may breed as a tadpole), 

 has cut out permanently its later stages and started a new course of 

 evolution from a young stage of an ancestor. This is known as neoteny, 

 and in it caenogenetic features may be taken up into the new adult 

 form: it may, for instance, account for some of the peculiarities of the 

 Larvacea (p. 679), the Cladocera (p. 362), and Leucifer (p. 415). A 

 young animal which is developing within an egg shell or in the womb 

 of its mother is known as an embryo : one which is fending for itself 

 is a larva. A stage which is larval in one animal has often become 

 embryonic in another. Embryonic development is said to be " direct." 

 Actually it is no more so than that which is larval. Caenogenetic 

 features are found in both, those which are most conspicuous being 

 in larvae organs of locomotion and feeding, in embryos the presence 

 of yolk or means of obtaining from the mother the nutriment which 

 the embryo cannot acquire from the outer world. 



