Structure and Development of the Nephridia. 351 



in tbese somites; but of the numerous larvae that I bave examined, 

 not one has presented tliis conditiou, so that its occurrence is at 

 auy rate exceptional. The pronephridia of somites IV aud V, on the 

 coutraiy, appear with perfect constancy as normal features of the 

 early development. 



In other respects the development of the post-nephridial somites 

 takes place (uutil the nineteenth somite is reached) in exactly the 

 same manuer as that of the more anterior somites as already 

 described. The result is the formation of a chaetigerons anterior 

 body-region which iucludes the first eighteen somites. The primi- 

 tive septa are, as above explained, dorsally incomplete, so that the 

 body-eavities commimicate freely with one another throughout the 

 greater part of this region. By the time of attainment of the nineteenth 

 somite it is foimd, however, that the four most anterior somites 

 bave become separated from the others by muscular partitions which 

 bave arisen at the posterior bouudaries of the first, third and fourth 

 somites. These structures are the fore- runners of the three 

 diaphragms of the adult; they do not represent the primitive 

 septa of this region, but are formed independently of these in a 

 manuer presently to be described. The bodj'-cavity from the poste- 

 rior bouudary of somite IV to the anterior boundary of somite XIX, 

 is entirely without septa, and remains in this condition throughout 

 lite, forming the spacious undivided coelom characteristic of the 

 Areuicolidae. 



With the appearance of somite XIX, a marked change takes place 

 in the character of the segmentation. The septum between so- 

 mites XVIII and XIX remains complete, forming a membrauous partition 

 which persists throughout life and divides the anterior chaetigerous 

 body-region from the region formed behind this limit. The somites 

 now formed are narrower aud shorter than those of the anterior 

 region (see Piate 24, Fig. 33); they are furthermore entirely desti- 

 tute of setae, and are separated from one another by complete 

 septa. At the beginning of the time of formation of this region, 

 its somites are not conspicuously narrower or shorter than those of 

 the anterior region. As growth proceeds, however, the characteristic 

 differences between the two regions become more evident, and are 

 well marked by the time the total number of somites has reached 

 the neighborhood of thirty. With a further increase in the number 

 of somites, the dividing line becomes better defined, until fiually, in 

 hirvae of 2 — 2.5 mm. length, the distinction between the two regions 



