WITH NOTES ON THE WEST INDIAN SPECIES. 



307 



its capacity of closed vesicle associated with the renal function would be physio- 

 logically comparable to the organ of Bojanus of the Molgulidae'. 



As for the collar-canals, since the essential organ of excretion is situated in the 

 proboscis it is obvious that some function other than that of excretion must be 

 assigned to them. Spengel gives good reason for supposing that the collar-canals 

 serve for the ingress of sea- water into the collar-coelom for the purpose of procuring 

 turgidity to assist in the peculiar method of locomotion of the Enteropneusta. 



It is possible that this also takes place through the proboscis-pores in certain circum- 

 stances and in certain species. But whereas the function of the collar-canals, whatever 

 it be, is constant, that of the proboscis end-sacs is as inconstant as they are variable. 

 In all species examined by me the end-sacs of the proboscis taper considerably towards 

 their internal (anterior) end. As we have seen, this tapering is sometimes carried so 

 far as to obliterate the coelomic opening of the sac. The collar-canals maintain their 

 calibre throughout their entire length and open invariably into the collar-coelom by a 

 wide semilunar funnel. 



It follows from what has preceded that both the proboscis-pores and collar-pores 

 are vestigial in respect of their primitive excretory function, but whereas the latter 

 have become definitely committed to an important and constant new function, the 

 former are in a completely unsettled state. Their only hope lies in their capacity 

 for forming new associations. 



The proboscis-pores and collar-pores of the Enteropneusta considered as homo- 

 dynamous structures may be contrasted as follows : — 



Proboscis-pores. Collar-pores. 



Highly variable. Kemarkably constant. 



Coelomic opening reduced or absent. Coelomic opening wide, semilunar and constant. 



Function impaired. Function true. 



It is a matter of great importance to note the definite fact that a proboscis 

 end-sac can persist as a coecal tube opening to the exterior after the loss of its 

 coelomic opening. The very fact that the end-sac persists under such circumstances 

 is surely evidence that its potentialities are not yet exhausted, and the outlook becomes 

 still more encouraging if it can become associated with the neuropore and with the 

 medullary tube. We have seen that this is a demonstrable possibility (cf PI. XXIX. 

 Fig. 17 A). 



Regional pore-canals can persist as vestiges after the loss of their coelomic openings. 

 Such vestiges, by acquiring neiu associations, may resume their physiological activity in 

 another sphere, by change of f motion ; or, they may be entirely superseded by a new 

 generation, i.e. by substitution, and so drag out the remainder of their existence in a 

 condition of vestigial degeneration. 



1 Spengel regards the anterior body-cavity (proboscis coelom) of Enteropneusta as an organ of the left 

 side, its dextral antimere being represented by the pericardium (Herzblase) [Mon. p. 681 et seq.^. Such extra- 

 ordinary complexity of structure and development as we have to deal with in the Enteropneusta is capable 

 of being regarded from different points of view. 



