Structure and Development of the Nephridia. 349 



region or pygidium) retains its eharacteristic peculiarities in an 

 alinost uualteied form tliroug-hout the whole of the development 

 (Figs. 7—10, Plates 22 and 23; 29—33, Plates 23 and 24). Its ecto- 

 dermal cells are small and vaeuolated, and contain a granular yellow 

 or brownish pig-ment, evidently of an excretory nature, which seems 

 to accumulate in this region as the animai grows older — possibly 

 because of its Isolation from the rest of the larvai body, and its 

 lack of vascular supply. A sharp line of demarcation exists betvveen 

 the ectoderm of this region and that of the growing zone immediately 

 anterior to it. The portion of the intestine contained in this region 

 corresponds to the proctodaeum, and is separated by a valve from 

 the entodermal portion immediately in front (Fig. 33, Piate 24). In 

 its ventral wall is inserted a bunch of strong stiff cilia. 



While the posterior portion of the larvai body is undergoing 

 elongation in this manner, difterentiation of the more anterior somites 

 is in progress. In young larvae (with fewer than nineteen somites), 

 each somite possesses at its earliest appearance — in addition to 

 its section of the intestine and the newly formed muscle-fibres of 

 the body-wall — the rudimeuts of dorsal and ventral setae; and 

 is separated from its neighbors by complete mesoblastic septal par- 

 titions (Figs. 9, 10; 29, 30, 31, 32). As growth proceeds, the 

 septa become incomplete dorsally, and the adjacent body-cavities 

 becorae continuous with one another. At the same time the setae 

 become fully formed and functional ; aud the supra- and sub-intestinal 

 blood-vessels, originally simple spaces between the opposed and 

 thinned-out walls of the early mesoblastic somites (Figs. 8, 9), be- 

 come well defined and of uniform diameter throughout. 



The stomach, as described above, is at an early stage (see 

 Figs. 7, 8, Piate 22) already sharply distinct from the Oesophagus. 

 Throughout the period now under consideration (from 8 to 18 so- 

 mites), the Oesophagus undergoes a continuai backward Prolongation 

 in a manner presently to be described, and its region of junction 

 with the stomach is thus gradually shifted backward from the second 

 (Fig. 8) to the seventh somite, its detìnitive position. In histological 

 structure, the walls of the Oesophagus consist chiefly of a single 

 layer of densely ciliated cubical epithelial cells. The stomach is of 

 greater diameter than the Oesophagus, and its w^alls, which are much 

 thicker ventrally than dor.sally, are composed of very eharacteristic 

 large uniformly stained cells of homogeneous appearance. Along its 

 ventral wall exteuds a ciliated groove continuous anteriorly with 



