190 MORPHOLOGICAL DEVELOPMENT. 



individuals are not reduced to the condition of segments of a 

 3omposite individual, and do not display any marked differ- 

 entiations ; yet there are some animals in which such 

 subordinations, and consequent heterogeneities, occur. The 

 oceanic Hydrozoa form one group ; and we have seen 

 reason to conclude that the Annulosa form another group. 

 It is not worth while, however, to occupy space in detailing 

 these unlikenesses of homologous segments, and seeking 

 specific explanations of them. Among the oceanic Hydrozoa 

 they are extremely varied ; and the habits and derivations of 

 these creatures are so little known, that there are no adequate 

 data for interpreting the forms of the parts in terms of their 

 relations to the environment. Conversely, among the^4??,- 

 nulGsa those differentiations of the homologous segments 

 which accompany their progressing integration, have so 

 much in common, and have general causes which are so ob- 

 vious, that it is needless to deal with them at any length. 

 They are all explicable as due to the exposure of different parts 

 of the chain of segments to different sets of actions and re- 

 actions : the most general contrast being that between the 

 anterior segments and the posterior segments, answering to 

 the most general contrast of conditions to which annulose 

 animals subject their segments ; and the more special con- 

 trasts answering to the contrasts of conditions entailed by 

 their more special habits. 



Were an exhaustive treatment of the subject practicable, 

 there should here, also, come a chapter devoted to the internal 

 structures of animals meaning, more especially, the shapes 

 and arrangements of the viscera. The relations between 

 forms and forces among these inclosed parts, are, however, 

 mostly too obscure to allow of interpretation. Protected as 

 the viscera are in great measure from the incidence of ex- 

 ternal forces, we are not likely to find much correspondence 

 between their distribution and the distribution of external 

 forces. In this case the influences, partly mechanical, partly 

 physiological, which the organs exercise on one another, 



