NEPHIIIDIA OF THE AOTLNOTliOOHA LARVA. 115 



attached at the free end of the tube^ near the nucleus, and 

 passes down the tube into the nephridial canaL In the case 

 of the larger Actinotrocha from Trincomalee, the nephriclium 

 gives off two or three short branches at its inner end, on 

 which are placed a very large number of solenocytes. The 

 nucleus is more spherical, but in other respects the soleno- 

 cytes differ but very slightly from those of the Sicilian larvse. 



CaldAvell's description, then, appears to be quite accurate 

 so far as it goes ; the flagellum working inside the " cylinder " 

 is really the only thing he missed. His comparison of the 

 nephridia of Actinotrocha Avith the nephridium of the larva 

 of Echiurus and the similar excretory system of Platy hel- 

 minths is most happy, and the facts supply another link in 

 the chain of evidence in favour of the view I have long been 

 supporting (6 and 7). 



Concerning the origin of the nephridia of Actinotrocha, the 

 views of previous authors are by no means in accord. It was 

 my intention, when I first undertook this work, to trace the 

 origin and fate of these interesting organs. Unfortunately I 

 have not been able to obtain the necessary material for a 

 complete history of their development. Others will, I hope, 

 soon be able to fill the gaps in our knowledge on this im- 

 portant subject. Caldwell believed the nephridia to arise 

 from the invagination at the " posterior pit." " I have 

 observed," he says, " mesoblastic cells, at the time when the 

 mesoblastic sacs of the trunk are forming, take the character- 

 istic shape of excretory cells with cylindrical processes 



I have failed to discover the origin of the main ciliated 



canal Dr. Hatschek believed that the whole organ 



was formed from the mesoblast cells mentioned above" (2). 

 De Selys Longchamps is uncertain as to the origin of the 

 nephridium (16). 



It is to Ikeda's excellent work that we must turn for the 

 most complete account of its development (9). This author 

 describes the origin of the nephridia from the invagination of 

 the epiblast (Caldwell's mesoblast) at the "posterior pit." 

 The invagination is said to sink inwards, to bifurcate, and 



