2 1 8 PI1 } 'L UM 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 Polychteta 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 Polychreta the Trochosphere stage is no 

 longer recognisable as such. 



A general Contrast of the Modes of Development in different 



Annelids. 



A. B. 



' ' Larval " Types, < ' Fcetal " Types, 



as in as in 



marine Chretopods, Earthworm, Leech, etc. 

 Polygordius, etc. 



Development indirect. Development direct, within egg 



A free-swimming Trochosphere capsule ; Trochosphere stage almost 



stage, with trunk almost or wholly or wholly suppressed. 



suppressed, with head region 



greatly developed, with adapta- f '' 



tions to free marine life. Litmbricits type Clepsine type 



with little nutri- with much nutri- 

 tive material in tive material in 

 ovum, with gas- ovum, with gas- 

 trula formed by trula therefore 

 imagination (em- formed by over- 

 bolic). growth (epibolic). 



GENERAL SURVEY OF THE CLASS CH^ETOPODA. 



I. Oligochgeta. 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 Oligochretes are 

 of much simpler structure. The most essential distinction from the 

 Polychreta is to be found in the complex reproductive organs. The 

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

 forms, such as Dero and Branchitira, 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 

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

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

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

 In general the Oligocheetes, 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 



