Structure and Development of the Nepliridia. 381 



subintestinal blood-vessel tbe two closely applied lamellae are usu- 

 ally seen to be slig-btly scpavated from eacb other wbere they pass 

 into continuity witb tbe peritoneal membrane. In tbe larva repre- 

 sented in section in Fig-. 9 the interlamellar spaee is continued for 

 a sligbt distance ontward from the lumen of the sub-intestinal 

 blood-vessel; this space represeuts in ali probability the beginning 

 of the segmentai blood-vessel which later passes to the body-wall 

 and traverses the dorsal lip of the nephrostome. The septum is seen 

 in this figure (and also in Fig. 10, 29, 31, 33, Plates 23 and 24) 

 to become continuous at its lateral margin with the lips of the 

 nephrostome. At its Insertion into the body-wall its cells become 

 continuous with those lining the body-cavity. 



The line along which the septum is inserted into the body-wall 

 is usnally considered to mark the boundary between two successive 

 somites. This line occupies in Arenicola a Constant and definite 

 Position with reference to the other segmentai structures of the 

 developing larva. Duriug the formation of the mesoblastic somites 

 the body-wall also undergoes a division into segmentai regions. 

 The metamerie structure of the ectoderm is most conspicuously 

 indicated by the seta e which appear at an early stage in the 

 intervals between the successive septa. The seta-sacs which give 

 rise to these structures appear laterally in the ectoderm in a position 

 slightly behind the middle of the interseptal region (Fig. 32, Piate 24). 

 The ventral ectoderm in the neighborhood of the nerve-cord also 

 early exhibits a metamerie structure, becoming slightly thinner or 

 constricted at segmentai intervals. The septa are inserted ventrally 

 along a transverse line passing imniediately in front of the con- 

 striction (Piate 23, Fig. 30). This line, therefore, if the above crite- 

 rion of segmentai limits is accepted, marks the boundary between 

 successive somites. Later in development, when the septum has 

 disappeared as such, the position of the line of demarcation corres- 

 ponds closely with that of the nephrostome and its associated brauch 

 of the segmentai blood-vessel — both of which structures, as already 

 stated, arise in intimate connection with the primitive intersegmental 

 septum. 



The septum, when first formed, constitutes a complete partition 

 between the body-cavities of adjacent somites. Soon after its forma- 

 tion, however, it becomes incomplete dorsally, as already mentioned 

 (p. 349), and for the remainder of its existence it is found only in 

 the ventral portion of the body-cavity as an incomplete membranous 



