TYPE ANNELIDA. 225 



to recognize two orders, Limicolce and Terricolce, aquatic forms being 

 referred to the former, and terrestrial ones to the latter a division, how- 

 ever, which is decidedly artificial. Less so, but still unsatisfactory, is a 

 division into Naidoinorp/ia, reproducing nou-sexually, and Lunibrieo- 

 iHu/'p/ia, reproducing by the sexual method only. It seems on the whole 

 better to omit a subdivision into larger groups, and recognize one into 

 families only. 



Development of the Oligochceta.-I.ii the development of the 

 Oligochseta there is practically no larval stage, 'but a sufficient 

 amount of nutrition is supplied to the embryo, either in the 

 form of yolk in the egg itself or as an albuminous substance 

 stored up in the interior of a cocoon in which the ova are 

 contained, to enable it to pass through all its early stages 

 while still within the egg-shell or cocoon, and to assume a 

 free life only when it has reached the form of the adult. The 



t 



Trochophore larva under such conditions is useless, and is 

 suppressed in the ontogeny, the development becoming thus 

 direct or of the foetal type. This mode of development has 

 been acquired as an adaptation to the aquatic or terrestrial 

 life, in which, for obvious reasons, the occurrence of a free- 

 swimming larva would be an inconvenience rather than an 

 advantage. 



In the Polychseta it was stated that usually at a very early 

 stage of development one cell, later dividing into two, differ- 

 entiates from the rest as the primary mesoblast, and gives 

 rise to all the mesodermal tissues of the adult worm. This is 

 an example of a precocious segregation of the mesodermal 

 material into a single cell. It is to be presumed that in more 

 primitive forms the mesoderm separated off from the endo- 

 derm only at a relatively late period of development ; the 

 tendency, however, for the appearance of an important struc- 

 ture to be thrown farther and farther back in the individual 

 development, to appear at successively earlier stages in the 

 development, has asserted itself to such an extent that the 

 mesoderm in the Polychseta makes its appearance while the 

 embryo is still composed of but a few cells, becoming there- 

 fore segregated in a single cell. Such a process has further- 

 more the advantage of permitting a rapid growth, the original 

 embryonic mesoblasts retaining their position at the posterior 



