WITH NOTES ON THE WEST INDIAN SPECIES. 



303 



Summary. 



1. My view is that the gill-slits arose originally as perforations in the inter- 

 annular grooves for the aeration of the gonads which occupied the dividing ranges. 



2. In the existing Enteropneusta the gill-slits no longer serve this primary 

 function directly, since there is an elaborate and highly peculiar vascular system. 



3. Consequently in the most primitive forms (e.g. Pt. flava) we find the gonads 

 quite removed from the neighbourhood of the gill-clefts and relegated to pleural 

 folds of the body. 



4. When secondarily restored to the main body-cavity owing to reduction and 

 disappearance of the pleural folds, there is a tendency for the gonads to restrict 

 themselves to a special region of the body, or in other words, to withdraw themselves 

 from the branchial region. 



6. This withdrawal of the bulk of the gonads from the branchial region manifests 

 itself in different ways : — 



(i) By reduction of the ramifications of the gonadic pouches in the branchial 

 region'. 



(ii) By removal from anterior end of trunk. 



6. The theory may be represented in tabular form as follows : — 



Gill-slits. 



Primary function = Oxygenation of gonads. 



Secondary function = Respiration of individual (Primary function superseded by elabo- 

 ration of vascular system). 



Primary position = Intergonadial. 

 Secondary position = Segmental. 



Primary number = Unlimited and indefinite. 

 Secondary number = Limited and definite. 



Gonads. 



Primarily coextensive with gill- slits. 



A. Unlimited. 



B. Limited. 



Secondarily emancipated from gill-slits. 



A. Restricted to genital pleurae. 



B. Restricted to genital region. 



' Besides Bal. canadensis already referred to, Bal. kupfferi is exceptional in that the gonads attain their 

 maximum development in the branchial region (Spengel). 



W. III. 



42 



