230 PHYLUM ANNELIDA. 



each side fuse above and below the gut to form the dorsal and ventral 

 supporting mesenteries of the gut. The head region is at first dispro- 

 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 Polychreta 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 Polychaeta the Trochosphere stage is no 

 longer recognisable as such. 



GENERAL SURVEY OF THE CLASS CKUETOPODA 



I. Oligochaeta. 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 Oligochsetes are 

 of much simpler structure. The most essential distinction from the 

 Polychseta 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 Branchiura, possess gills of simple structure, 

 while the West African Alma nilotica 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 AZolosoma, 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 

 forms ; of these most familiar are Tiibifex rivulorum, often found 

 in the mud of brooks, and the species of Nais, remarkable for their 

 power of asexual budding. Some Microdrili live between tide-marks. 

 The leech-like Branchiobdella y which is parasitic on the gills of the 

 fresh-water crayfish, is a somewhat aberrant member of the group. 

 The Megadrili include the larger Oligochsetes, mostly living in earth, 

 and commonly designated as "earthworms." The largest form is a 

 Tasmanian species (Megascolides gippslandicus) y measuring about 6 ft. 

 in length. 



II. Polychseta. As contrasted with the more or less subterranean 

 earth- and mud-worms, the marine Polychoeta have a richer develop- 

 ment of external structures and a more complex life history. The 

 external appearance is greatly modified by the relative degree of 

 development of the parapodia, which are lateral outgrowths typically 

 functioning as walking "legs," or as swimming organs. A para- 

 podium, when fully developed, is divisible into a ventral neuropodium 

 and a dorsal notopodium. Each of these is bilobed, bears a tactile 

 process or cirrus, and is fringed with firm bristles or setae. Within the 

 substance of each lobe is embedded a stout needle-shaped " aciculum," 



