100 MORPHOLOGICAL DEVELOPMENT. 





for ideologically. That certain organs of nutrition and re- 

 spiration and locomotion are repeated in each segment of a 

 dorsibranchiate annelid, may be regarded as functionally ad- 

 vantageous for a creature following its mode of life. But 

 why should there be a hundred or even two hundred pairs of 

 ovaries? This is an arrangement at variance with that 

 physiological division of labour which every organism pro- 

 fits by — is a less advantageous arrangement than might have 

 been adopted. That is to say, the hypothesis of a designed 

 adaptation fails to explain the facts. Contrariwise, 



these structural traits are just such as might naturally be 

 looked for, if these annulose forms have arisen by the in- 

 tegration of simpler forms. Among the various compound 

 animals already glanced at, it is very general for the united 

 individuals to repeat one another in all their parts — repro- 

 ductive organs included. Hence if, instead of a clustered or 

 branched integration, such as the Ccelenterata, Polyzoa and 

 Tunicata exhibit, there occurs a longitudinal integration; we 

 may expect that the united individuals will habitually indi- 

 cate their original independence by severally bearing germ- 

 producing or sperm-producing organs. 



The reasons for believing one of these creatures to be an 

 aggregate of the third order, are greatly strengthened when 

 we turn from the adult structure to the mode of develop- 

 ment. Among the Dorsibranchiata and Tubicolce, the em- 

 bryo leaves the egg in the shape of a ciliated gemmule, not 

 much more differentiated than that of a polype. As shown 

 in Fig. 162, it is a nearly globular mass; and its interior 

 consists of untransformed cells. The first appreciable change 

 is an elongation and a simultaneous commencement of seg- 

 mentation. The segments multiply by a modified gemma- 

 tion, which takes place from the hinder end of the penultimate 

 segment. And considerable progress in marking out these 

 divisions is made before the internal organization begins. 

 Figs. 163, 164, 165, represent some of these early stages. In 

 annelids of other orders, the embryo assumes the segmented 



