RECENT WORK ON THE PROTOCHOBDA. 225 



The justification for the new collective name, Branchiotrenia, 

 is contained in my forthcoming memoir, where it is introduced 

 in connection with a new theory of gill-clefts, to which I have 

 been led by my study of the Enteropneusta. This theory may 

 be barely and briefly stated as follows : 



The gonads and gill-slits were primarily unlimited in 

 number and co-extensive in distribution, the gonads having a 

 zonary disposition and the gill-slits occupying the interzonal 

 depressions. 



The primary function of the gill-slits was the oxygenation 

 of the gonads, their secondary function being the respiration 

 of the individual — the change of function having taken place 

 pari passu with an elaboration of the vascular system. 



Correlatively with the progressive regional differentiation of 

 the body, the gonads and gill-slits became limited both ante- 

 riorly and posteriorly. The anterior limitation of the gill- 

 slits behind the collar region is constant in all Enteropneusta, 

 but the posterior limitation is excessively variable.^ The 

 emancipation of the gonads from their topographical relations 

 with, and functional dependence on, the gill-slits has taken 

 place in several ways, but the resultant tends to be, and 

 eventually actually is, the restriction of the gonads to a post- 

 branchial genital region. 



Such, in outline, is the theory to which I have committed 

 myself. The Harmer-Brooks-Masterman theory of gill-clefts 

 does not, in my opinion, account for their prime origin, but it 

 does perhaps explain the retention of a single pair of gill- 

 clefts in forms like Cephalodiscus and Appendicularia. 



In recording the fact that Spengel divided the species of 

 Enteropneusta into four genera, MM. Delage and Herouard 

 prefer to describe them "en bloc sous leur ancienne denomi- 

 nation commune." This conservative method of treatment is 

 somewhat foreign to the general spirit of the book, since an 

 endeavour has been made to incorporate the results contained 

 in the most recent publications. As a consequence, some 



' Both anterior and posterior limits oi' tlie gonads are variable. 



