490 Cresswell Slie.'irer 



and nephridium, are foiind side by side. Once this exists, various 

 niodificd couditious arise iu wliich one of these orgaus replaces the 

 other, or we get a partial fusion of the two structures. It is only on 

 this theory that we can explain the complex orgaus of some Poly- 

 chaets, consisting in a genital funnel grafted on the end of a nephri- 

 dium. It is to be hoped therefore that the study of the develo])- 

 ment of the solcnocytes will furnish fresh evidenee agaiust that 

 theory which still regards the trae nephridium as a modified but 

 an essential part of the coelom. 



Part I. Phoroiiis. 



1. Literature. 



The conflicting Statements to which the study of the developmeut 

 of Phoronis has given rise, are gradually decreasing as new facts 

 are brought to light in its life-history. As the result of much of 

 this recent work, the tendency of opinion ma}^ now be said to 

 regard the adult, rather than the Actinotrocha larva, as the most 

 suitable place to look for what chordate features Phoronis ma 

 possess. Eecent work has shown how much Mastermax (1?, 18) 

 has been influenced by theoretical prejudices in bis conception of the 

 structure of this interesting animai. Yet the concurrent work of 

 Ikeda, Güodeicii, De Selys Longchamps and Cowles has never- 

 theless shown that thcre are at least three body-cavities in the larva, 

 of which two are uudoubtedly coelomic. This taken witli the 

 Trochophore characters possessed in the early stages, make it a 

 curious puzzle. The more one considers these early stages up tili 

 the time the larva begins to assume the Actinotrochal stage, the 

 more one is impressed by the Trocho[)horal characters it possesses ; 

 the more one examiues it after this stage is reached, the more one 

 is intluenced by the many features it has in common with the 

 Brachiopoda and the Hemichordata. Conklin (5, ])ag. 70) in a recoiit 

 papcr has gone even so far as to say tliat with the exception of 

 the segmented coelom, there is no esscutial dill'ercnce between tlie 

 Actinotrocha larva and the Brachiopod larva, but Conklins conclusion 

 was based on the work of Master.man, wlio is mistaken in regar- 

 ding the preorai coelom of Phoronis like that of lìrachiopods 

 a gut diverticulum. There are evidences in the structure of the 

 adult Phoronis however to warrant the suggcstion first put forward 



