140 INVEETEBEATA CHAP, vn 



closed by the union of the four cells 5c 12 , 5cl 12 , 5c u , and Sd 1 

 respectively. At a later period of development, however, the 

 permanent anus re-opens at the same spot, so that the temporary 

 closure is an event of no importance. 



We have thus the problem solved before our eyes hoiv, out of a 

 single primitive opening used both for injestion and egestion or defaeca- 

 tion, such as we find in Coelenterata and Platylielmintlies, separate 

 openings for injestion and egestion were formed. 



When the primitive anus is closed the blind end of the gut 

 remains in close contact with the cells 4d r and 4cV. Two outer 

 columns of cells parallel to the first two are then formed. These 

 consist on the right side of 2c 222a , 2c 222 P, 3c 2a2 , and 3c 2 P aa , and on the 

 left side of 2a 222a , 2a 222 P, 3d 2a2 , and 3d 2 P aa . The hinder cells of these 

 two outer ridges also meet; i.e. first 3c 2a2 and 3d 2a2 , then 2a 222 P and 3c 222 P, 

 and lastly 3c 2paa and 3d 2 P aa ; but their front cells, 2c 222a and 2a 222a , do 

 not meet ; they, as we have already seen, help to form the sides of 

 the stomodaeum (Fig. 104, B). 



As these outer columns of ectodermal cells meet, the endodermic 

 pouch shrinks away from them and leaves a blastocoelic space 

 between it and them ; so that the process of closing the ventral wall 

 of the gut is completed before the ventral ectoderm is complete, and 

 thus, for a brief moment, the blastocoele is actually in open com- 

 munication with the external world (Fig. 105, C). 



The final closing of the outer part of the blastopore is effected 

 by the rotation inwards and backwards of the cells 3c' 2 P aa and 3d 2 P aa . 

 These cells rotate through an angle of 180, and so come to lie actually 

 within and behind the cells 3c 2 P a P and 3d 2 P a P (Fig. 104, I)). 



It is at this stage of development that the cells 3c 2 PP a and 

 3d2ppa W ander like amoebocytes into the blastocoele and form the 

 solenocytes of the two archinephridia. The pre-anal tuft of cilia, 

 the telotroch, is formed by the cell 3d 2 P aa . The lower lip of the large 

 mouth is formed by the cells 3c la , 3d la , 3c 2al , 3d 2al , 3c 2a2 , 3d 2a2 , which 

 swing through a right angle to occupy that position. The cells 3c lp , 

 3d lp , become elongated in an antero-posterior direction, acquire short 

 cilia, and form the metatroch, i.e. the circular band of feebler cilia, 

 which runs parallel to the prototroch behind the mouth (Fig. 104, D). 



Turning our attention now to the second quartette in quadrant 

 D, we find that the cell 2d 222 wanders like an amoeba over the ventral 

 surface of 4d r and 4dl Each of these cells has by this time budded 

 off a small anterior cell, 4d rl and 4d'\ which is the beginning of the 

 adult mesoderm on each side. 



We find now, when we look at the under side of the Trochophore 

 behind the mouth, two large thin plate-like cells 3c 2pap and 3d' 2pap in 

 front. These constitute what Woltereck calls the hyposphere, or 

 under surface of the almost spherical larva. To the sides of these, 

 lie 3c 2p P p and 3d 2p P p , the tubal cells of the archinephridia. Behind 

 them are a group of three compact cells covering the adult mesoderm ; 

 and these cells which, for reasons to be explained later, we call the 



