80 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



Larva VI A, Jfi mm. 



The nephrostomal cavity is now still more completely cut off from the 

 glomerular cavity than in Larva V. Figure 42 shows the section in which 

 it becomes confluent with the glomerular cavity. The thickened disk 

 which was seen in its ventro-lateral angle in the preceding larva 

 (Fig. 41, evg. 1) is now seen to have evaginated to form a thick- walled 

 pit {evg. 1, Fig. 42). In the evagination figured (from the right side of 

 the body) there is a tendency toward a spiral coiling, a common but not 

 universal feature in these anterior evaginations. The open lumen does 

 not extend very far into the cell-mass, but the direction of its continua- 

 tion is indicated by the arrangement of the nuclei. At the deep end 

 of the spiral arises a duct (Fig. 43, dt. i), which runs caudad for some 

 sections, dwindles to a cord, and disappears (Fig. K, dt. 1). At the 

 point of origin of '' duct 1 " a cord starts cephalad and continues for 

 several sections. For the sake of convenience I shall call this the pi'e- 

 coelomic duct (dt. pr^coeL, Fig. K). Its position at a later stage is shown 

 in Figure 48, dt. pr^coel. 



On the left side of the body, the first evagination is similar to that on 

 the right side, with the exception that it arises more dorsally, from the 

 middle of the lateral wall of the nephrostomal cavity (in a position corre- 

 sponding to that of the thickening shown in Figure 41), and is a deep, 

 straight pit with a wide lumen, much like the one shown in Figure 49. 



From the mouth of the evafdnation the thickened band runs back 

 (Fig. 43, tae. e'th. a) and out on the shelf, just as in previous stages, 

 there to be joined by the band from the second or posterior nephro- 

 stome. The local thickening of the band from the second nephrostome 

 is very conspicuous, but has not as yet evaginated. Figure A' (p. 88) 

 represents diagrammatically the condition of the evagination, etc., at 

 this stage. 



Larva VI B, 4-2 mm. 



The anterior evagination on one side of this larva is similar to those in 

 Larva VI A. On the other side it is very small, whether just arising or 

 already degenerating, it is impossible to decide. An important advance 

 over the preceding larva has been gained in that the thickened disk ven- 

 tral to the second nephrostome has also evaginated both on the right and 

 on the left sides of the body. These posterior evaginations, which, as 

 will appear later, are the permanent ones, are not so deep as the anterior 

 ones. Figure 44, evg. 2, shows that of the left side of the body and is 

 drawn to the same scale of magnification as the anterior evagination 



