144 INVEETEBRATA CHAP. 



blastopore and later of the primitive anus, it will be seen that the 

 permanent anus corresponds to the hindermost piece of the primi- 

 tive one. 



The Trochophore is now complete and can begin to feed. It is 

 the only larva in which the ancestry of every cell has been 

 completely worked out, and hence its cell-lineage has been given with 

 a completeness which it will not be necessary to repeat in the case of 

 other forms. As now constituted it is very similar to the Pilidium, for 

 the ventral surface is still almost flat and no trace of the projection 

 which is to form the body of the future worm, has yet appeared. It 

 is true that the Pilidium possesses no anus, but this, as we have seen, 

 is also true of the young Trochophore. 



The distinctive features of the Trochophore at this stage are the 

 metatroch, the archinephridia, and the telotroch, all of which are 

 wanting in the Pilidium. 



FURTHER DEVELOPMENT OF POLYGORDIUS 



There are two species or varieties of the European Polygordius, 

 one found in the North Sea, P. lactens, and one in the Mediterranean, 

 P. appendiculatus. The segmentation of the egg and the early 

 development up to the attainment of the Trochophore stage is 

 strikingly similar in both varieties, but the later development, till 

 the attainment of the adult form, is very different in the two cases. 

 The adult worms according to Woltereck are practically indistinguish- 

 able from each other, so that we have here a curious instance 

 of specific peculiarities being developed during the later larval 

 history. 



We shall describe the later development of the Neapolitan species 

 first, as it is the simplest. The external features of this stage of 

 development were exhaustively described by Fraipont (1887), but its 

 true significance has only been made clear by Woltereck (1902 and 

 1905). In this species, after feeding has gone on for some time, a 

 rapid period of cell division and growth sets in in the three cells 

 forming the trunk blastema (these it will be remembered are 2d~ 

 and 3c 2paa and 3d 2 P aa ) ; and in two cells, 4d rp and 4d lp , the mother cells 

 of the adult mesoderm which lie internal to the three cells just men- 

 tioned. These two latter cells had already, in the Trochophore, each 

 budded off a small cell (4d rl and 4d n ) in front of them : this process 

 is now repeated many times, and in this way two long strings of 

 cells, the mesodermic bands, are formed. The original mother cells 

 of the bands are termed teloblasts. 



The multiplication of the cells of the trunk blastema furnishes 

 new ectoderm to cover these bands, and in this way a post-trochal 

 outgrowth of the body is formed. The intestine elongates at the 

 same time by the growth and division of the cells forming its walls, 

 for the anus is situated at the termination of this post-trochal 

 projection. 



