84 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



a lower level than the nephrostomes. Instead it lies at the same level, 

 or even more dorsally than they. As in the preceding larva, the duct 

 from the anterior evagination disappears in that part of its course which 

 is anterior to the second evagination, bnt the duct from the latter is in 

 places double, showing that it has been formed by a fusion of the two 

 ducts. The posterior duct on both sides of the body becomes curiously 

 attenuated a short distance behind its origin, so that it is reduced in 

 cross-section to two or three cells, but is always clearly marked off from 

 the surrounding tissue by its sheath.-^ Farther back it again becomes 

 a well-defined tube, which extends for some distance alongside the 

 Wolffian duct. Figure M, page 88, illustrates this and the next stage. 



Larva /X, 50 mm. — Gills reduced to stumps. 



This larva is interesting from the fact that, although older than any 

 of the preceding, the duct from the anterior evagination (both right and 

 left) has not suffered any degeneration. It {dt. 1, Fig. 51, Plate 4) 

 passes the second evagination and, after running parallel to the duct 

 from the second evagination for some distance, approaches and fuses 

 with it. The beginning of this fusion is shown in Figure 50 (dt. i, dt. 2). 

 The single duct thus formed continues caudad for some distance along 

 the Wolffian duct. On one side of the body it ends independent of the 

 Wolffian duct and enclosed in its own sheath. On the other side it 

 approaches the Wolffian duct until the two are enclosed in a common 

 sheath and then, applying itself to the Wolffian duct, disappears appar- 

 ently by fusing with it. Figure 51 shows the thickened band {tae. eHh. a), 

 which passes back from the first evagination, around the posterior edge 

 of the shelf (marg. p. tab.) and forward again in the sub-glomerular cavity 

 (tae.e'th.y).'^ Two sections behind the one figured, the shelf separates 

 along the line marg. p. tab. from the lateral body-wall. Where this 

 occurs, the two portions of the thickenings tae. eHh. « and ta£, eHh. y' are 

 seen to be continuous. In the same section the ridge containing the 

 Wolffian duct and fundaments of the Mtillerian duct becomes well sepa- 

 rated from the lateral body-wall (as in Figure 50) by an infolding, whose 

 position is indicated in Figure 51 by the groove sul. 



1 In poorly preserved specimens this attenuated part of the duct might easily 

 be overlooked. This condition may account for the conception of some authors 

 that the duct is not developed continuously witli the ostium. 



2 By comparing Figure / with M it will be understood that the only part of the 

 epithelial band now remaining on the upper side of the shelf is that designated by 

 a, 7 having merged with y' by being brought beneatli the shelf. 



