ON THE DIPLOOHORDA. 281 



On the Diplochorda. 



1. The Structure of Actinotrocha. 



2. The Structure of Cephalodiscus. 



By 



A. T. Masterninii, B.A., B.Sc, 



Lecturer and Assistant Professor of Natural History in the University 



of St. Andrews, N.B. 



With Plates 18-26. 



Introduction. 



Amongst the many and diverse types of animal structure 

 which are familiar to the zoologist there are some, both- simple 

 and complex in character, which have never offered any serious 

 difficulty with regard to their systematic position in the natural 

 classification ; and there are others which ever since their first 

 discovery have provided material for discussion concerning 

 their systematic relationships, and whose true genetic con- 

 nection has not yet been clearly elucidated. From the nature 

 of the case these latter animals are usually generalised types, 

 and a certain proportion of the difference of opinion has arisen 

 by diflferent observers laying greater stress upon special struc- 

 tural features. 



On the other hand, possibly on account of the doubtful 

 nature of their genetic relationships, these animals are com- 

 monly either left out of zoological text-books altogether, or 

 are merely referred to casually as incertse sedes, although 

 the study of their structure and development always gives the 

 promise of renewed light upon the many morphological pro- 

 blems of the day. In other words, these animals form the 

 anomalies or exceptions which in other sciences, as well as 



