202 SEGMENTED WORMS OR ANNELIDA. 



portionately large, but later, by an independent process of growth, 

 becomes reduced. The larva abandons its pelagic life, and becomes 

 adult. 



Comparing the development of Polychfeta with this, we find that the 

 Trochosphere is often modified, and that segmentation tends constantly 

 to appear at an earlier stage. As a further step in the same direction, 

 we may note that in some Polychseta the Trochosphere stage is no 

 longer recognisable as such. 



A general Contrast of the Modes of Development in different 

 Annelids. 



A. 

 ' ' Larval " Types, 



as in 



marine Chaetopods, 



Polygordins, etc. 



Development indirect. 



A free-swimming Trochosphere 

 stage, with trunk almost or wholly 

 suppressed, with head region 

 greatly developed, with adapta- 

 tions to free marine life. 



B. 



" Foetal" Types, 



as in 



Earthworm, Leech, etc. 



Development direct, within egg 

 capsule ; Trochosphere stage almost 

 or wholly suppressed. 



Lumbricus type 

 with little nutri- 

 tive material in 

 ovum, with gas- 

 trula formed by 

 invagination (em- 

 bolic). 



Clepsine type 

 with much nutri- 

 tive material in 

 ovum, with gas- 

 trula therefore 

 formed by over- 

 growth (epibolic). 



General Survey of CHiETOPODA. 



I. Oligochseta. — The general characters may be gathered from 

 the description of the earthworm, but it is to be noticed that the earth- 

 worms are specialised forms, and that the fresh-water Oligochaetes are 

 of much simpler structure. The most essential distinction from the 

 Polychaeta is to be found in the complex leproductive organs. The 

 absence of gills, though general, is not universal, for a few fresh-water 

 forms, such as Dero and Branchiura, possess gills of simple structure, 

 while the West African Alma has more complex branched retractile gills. 

 Among other characters may be noticed the tendency to variation in 

 the structure of the excretory system. In all, with the exception of 

 Alolosoma, certain of the nephridia are modified to serve as genital ducts, 

 while in the Megascolicidse the nephridia tend to be reduced to a mass 

 of minute tubules ramifying over the inner surface of the body-wall. 

 In general the Oligochaetes, however, show more uniformity of structure 

 than their marine allies. 



They may be divided into two main groups — (i) the Microdrili, 

 and (2) the Megadrili. The first group includes the small aquatic 



