VII 



ANNELIDA 



145 



hb 



ptr 



By longitudinal divisions the mesodermic bands become several 

 rows of cells thick, and a faint indication of the division of the post- 

 trochal "body" into segments is now discernible. These segments 

 are indicated in the ectoderm by faint transverse grooves parallel to 

 one another; in the mesodermic bands by the appearance in each 

 band of a set of cavities, which we may term somites, situated 

 one behind the other, corresponding to the grooves in such a 

 way that, in each segment, 

 one pair of cavities is formed. 

 The right and left somites in 

 each segment rapidly expand 

 and displace the blasto- 

 coele, and finally meet one 

 another both above and below 

 the intestine. Then, where their 

 walls impinge on one another, 

 absorption takes place, and so 

 instead of two coelomic sacs one 

 coelomic ring-shaped space is 

 found in each somite. 



Whilst these changes have 

 been taking place the archine- 

 phridia have undergone further 

 development, by the adhesion to 

 them of further cells budded 

 from the third quartette. The 

 original pore cell divides and 

 gives rise to a string of cells. 

 The original flame cell, or soleno- 

 cyte, persists, but the newly- 



oes 



added cells form additional 

 solenocytes. The transformed 

 nephridia are now known as the 

 first pair of protonephridia 

 (P.N 1 , Fig. 108, A). Each consists 

 of a tube which forks internally 

 into two branches, and each 



t.tr 



FIG. 107. Later stage in the development of 

 Polyg&rdius appendiculatus, in which the 

 " worm-body " is being formed by the 

 growth of the trunk blastema. (After 

 Woltereck. ) 



Letters as before. In addition, a, permanent 

 anus; h.b, hind limit of head blastema; int, intes- 

 branob tprini iarp<?inrwnrppnliflr tine; P ' N ' protonephridium ; oes, oesophagus; st, 



n terminates in two pecunai stomach . tb> front limit of trunk blastema, 

 solenocytes. Each of these 



solenocytes consists of a head studded all over with blind cuticular 

 tubes, each of which contains a flagellum (sol 1 , Fig. 108, A). Behind 

 this pair of protonephridia a second pair arises. Each of these 

 consists of a tube opening by a pore on one side in the region of 

 the first somite formed from the trunk blastema. The tube is 

 terminated internally by solenocytes which are situated in the 

 swollen body of the Trochophore, and which are derived from 

 wandering cells, descendants probably of the third quartette. The 

 tube itself owes its origin to a pore cell situated in the ectoderm of 

 VOL. i L 



