WITH NOTES OX THE WEST INDIAN SPECIES. 305 



pouch to be taken into consideration. When this has once been recognised it is 

 allowable to use the expression " anterior neuropore," in the wider sense, to cover 

 the two structures which, in the Enteropneusta, usually combine to produce it, but 

 which sometimes assert their independence 1 . 



Spengel {Man. p. 470) points out that where two proboscis-pores occur in the 

 adult it is highly probable that the dextral pore has a secondary origin in an 

 ontogenetic sense, as no Toruaria has ever been found with two water-pores. 



This is probably true for those forms which pass through an indirect development. 

 Thus we are, for example, bound to assume that the Tornaria of Pt. fiava possesses 

 a single sinistral pore. With those Balanoglossidae which possess two pores, namely, 

 Bal. kupfferi and Bal. canadensis, the matter is different and I should be prepared 

 and even expect to Hud that in those species, having a direct development, the two 

 pores would arise together as they do in regenerating specimens of Pt. fiava. It is 

 sufficiently clear that the paired condition of the pores is phyletically the more 

 primitive, and Spengel intimates that he is likewise of this opinion. We have there- 

 fore here a very interesting example of a phyletically older condition being recapitu- 

 lated as a secondary ontogenetic phenomenon 2 (cf. above Pt. carnosa). 



The comparative morphology of the proboscis-pores is a subject of almost dis- 

 couraging complexity; and the attempl to elucidate it makes no light task. 



Three facts, inter alia, which have come under my observation, have conducted 

 me to certain ideas which, so far as they go, are quite clear and definite in my 

 own mind. These facts are : — 



1. The terminal tubular vesicle or end-sac w r hich typically opens internally 

 into the proboscis coelom and externally by the proboscis pore, may be quite shut 

 off from any communication with the coelom ; in other words, it may be completely 

 emancipated from the coelom (Pt. fiava, Pt. jamaicensis). 



2. The end-sac 3 may have a comparatively long post-trematic coecal extension 

 (Pt. carnosa, Pt. biminiensis, Pt. jamaicensis, Sp. alba). 



3. The end-sac may open into the medullary tube behind the anterior neuro- 

 pore (Pt. carnosa). 



If, by a legitimate mental abstraction, we reflect upon the condition in which 

 there is a coecal sac opening into the medullary tube which, in its turn, opens to 

 the exterior by the neuropore, we have before us essentially the combination met 

 with in the Ascidian larva with the difference that, in the latter, the neuropore does not 

 open directly to the exterior but into the dorsally placed mouth. 



1 The structure described by Masterman in a species of Actinotrocha as a neuropore is what Spengel 

 would rigbtry call an "Epidermistasche," and is certainly not a neuropore in the strict sense. (A. T. Master- 

 man, "On the Diplochorda," Q. J. M. S., Vol. 40, 1897, p. 281.) 



- Although paired water-pores have never been demonstrated to exist in Tornaria, they have been observed 

 to have a normal though transitory existence in certain Echinoderm larvae, by Metschnikoff, Brooks, Field and 

 Grave. (See Caswell Grave, "Embryology of Ophiocoma echinata Agassiz," Johns Hopkins Univ. Circ, Vol. 18, 

 Nov. 1898, p. 6.) 



3 By this term I shall, in the remarks which follow, refer to what Spengel calls the " Eichelpforte." 



42—2 



