ox DEVELOPMENT ETC. OF PHOP.OXIS. 531 



two iiioro prominent ridsio?^ running on oaeli sitio obliquely ant- 

 eriorly. The preoral lobe grows rapidly so as to hang down on the 

 ventral side and as a consequence of this an œsophageal canal is 

 formed (fig. 37, œs). The œsophageal wall is, therefore, ecto- 

 blastic in origin and is composed of strongly ciliated columnar 

 cells. About this period the nephridial invagination l)ecomes 

 completely divided into two lobes at the proximal end, as T 

 have already described (figs. 37 and 38, 7irp. p.). In more ad- 

 vanced larvte, the pit is split throughout its entire length into 

 two nearly parallel canals, each of which opens independently to 

 the exterior. Figs. 39 a-c show tln-ee transverse, though not 

 consecutive, sections Classing through the posterior region of a larva 

 at such a stage. In the first of these figures, the two cell-masses 

 {nep. c.) on either side of the stomach represent the uppermost 

 portion of the nephridial canals. In the second figure, each of the 

 cell-masses encloses an easily distinguishable lumen. The two 

 canals finally open to the exterior each Iw a small pore {nep. o.), 

 as seen in the third figure (only one pore is cut through in the 

 al)Ove figure, the section l)eing slightly oblique to the main axis 

 of the Lirval body). In the above figure we see an ectoblastic 

 cell-mass separating the right and the left nephridinl canals (nep. c). 

 How is this partition 1)rouglit al)Out ? I think it is caused bv re- 

 evaginntion of the distal unpaired portion of the nephridial pit, as 

 by that process the pit wall forming the above portion is graduallv 

 transferred to the body-surface of the larva. 



Meanwhile the oesophagus becomes more and more elongated, 

 while the paired tentacular thickenings bulge out each into two 

 perceptil)le prominences. The latter represent the rudimentary state 

 of two larval tentacles, each of which has internally a cavity con- 

 tinuous with the ])ostoral body-cavity. Fig. 10 represents a larva 



