306 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



The theory, as to the broad truth of which I am myself quite convinced and for 

 which I will proceed to produce the available evidence, may be stated briefly as follows : — 

 The proboscis pore^ of the Enter opneiista is represented by and is homologous with the 

 inner or cerebral opening of the neuro-hypophysial apparatus of the Ascidian larva ; 

 the end-sac of the Enteropneusta typically communicates internally with the coelom, but, 

 within the limits of the group, we find signs of its emancipation'^ from the coelom; the 

 hypophysial canal of the Ascidian larva has no relations with the body-cavity but it opens 

 at one end into the medullary tube (cerebral vesicle) and at the other into the branchial 

 sac at the base of the buccal cavity. Thus a special significance is given to the pecidiar 

 mode of origin of the Ascidian subneural apparatus (gland and duct) and an explanation 

 is forthcoming as to the apparent absence of anything like a proboscis-pore in the Ascidian 

 larva^. 



a. Evidence of change of function ; Excretory system of Enteropneusta. Apart from 

 the evidence as to change of function, or loss of previous function of the proboscis- 

 pores which is furnished by the fact of their greater or less emancipation from the 

 coelom, there is also evidence of another kind. The proboscis-pores are clearly homo- 

 dynamous with the collar-pores and the truncal pores (presumed vestiges of which 

 occur in Spengelia). It is to be supposed that these three pairs of regional pores 

 represent the primitive excretory canals of what Masterman (1897 loc. cit.) has called the 

 archimeric regions of the body. But they no longer function as excretory canals since 

 the function of excretion has been relegated to the glomerulus (proboscis-gland of 

 Bateson) which is a structure sui generis. It might be supposed that the proboscis- 

 pores would at least carry off the products of excretion resulting from the physio- 

 logical activity of the glomerulus, and it is possible that this does occasionally happen. 

 But if it were their essential function it should invariably happen. But it does not. 

 In Bal. canadensis Spengel has found that the proboscis-pores are quite vestigial and 

 in Pt. flava, as described above, the communication between the end-sacs and the 

 proboscis coelom is sometimes occluded and sometimes quite obsolete. 



It will be asked what becomes of the products of excretion if they are not 

 discharged to the exterior, and the answer is that it is not absolutely necessary, in 

 animals of the grade of organization of the Enteropneusta, that excretory products 

 should be removed from the body (e.g. Ascidians). In close topographical relation with 

 the glomerulus is a capacious vesicle closed on all sides, called the pericardium 

 (Herzblase) on account of its relations to the central blood-space. 



The endothelium of this so-called pericardium is subject to remarkable proliferation 

 which varies greatly in its amount in different individuals (perhaps at different periods 

 in one individual). It is quite reasonable to suppose that besides its topographical 

 relations to the vascular complex known as the glomerulus it possesses functional 

 relations with that organ. If this be so, the pericardium of the Enteropneusta in 



1 I say nothing as to dextral or sinistral pore or both. 



2 Apropos see also Spengel on Bal. canadensis and Bal. kupfferi {Mon. pp. 472 — 473.) 



^ Of. A. Willey, " Studies on the Protochordata. II. The development of the neuro-hypophysial system in 

 Ciona intestinalis and Clavelina lepadiformis." Q. J. M. S., Vol. 35, 1893, p. 295. 



