WITH NOTES ON THE WEST INDIAN SPECIES. 



311 



as might appear. All these structures in animals of the grade of organisation of the 

 Enteropneusta, Cephalochorda and Urochorda are in a more or less primordial condition, 

 and hence appear deceptively simple just as an egg-cell which conceals the potentialities 

 of the future organism may appear a simjile matter. 



In the Enteropneusta there are, as we have seen, dorsal canalicular portions of the 

 proboscis coelom separated from one another by the pericardium. Each of these dorsal 

 coelomic canals may, but usually only one does, open into a tubular end-sac, which in 

 turn opens to the exterior. Bateson found in Bal. kowalevskii that the end-sac arises 

 as an ectodermal ingrowth, and Spengel has found the same in regenerating specimens 

 of Pt. minuta. 



We have to consider therefore the possible and particular fate of 



1. The dorsal coelomic canals in their capacity as portions of the proboscis coelom 

 or anterior body-cavity. 



2. The opening of a coelomic canal into an end-sac, which is equivalent to the 

 opening of the coelom at an ectodermal surface. 



3. The end-sac itself. 



4. The external orifice of the end-sac. 



With regard to the fate of the anterior body-cavity there is one remote though 

 instructive ground of comparison between the Enteropneusta and the Cephalochorda. 

 In the larva of Amphioxus the larger or dextral portion of the head-cavity (usually 

 called the right head-cavity) forms the cavity of the snout or rostrum. In the adult 

 this cavity is lost in the massive development of the laminar tissue (Pouchet) which 

 is characteristic of Ampliioxus. Similarly in the Enteropneusta the posterior ends of 

 the coelomic canals of the proboscis lose themselves in and contribute cellular islets to, 

 the chondroid tissue (Marion, Spengel). 



It is very important to remember that in dealing with these structures there 

 are two kinds of pores to be accounted for and not one pore only (above Nos. 2 

 and 4). In the path which has culminated in the Urochorda the coelomic opening 

 (above No. 2) has, I believe, demonstrably vanished. It may not have vanished in 

 Amphioxus ; it may have had there another fate, a change of destiny instead of an- 

 nihilation. 



The praeoral pit of the larva of Amphioxus is a portion of the coelom which 

 opens to the exterior, that is to say, which opens at an ectodermal surface. It has 

 been the custom to speak of the right and left head-cavities of Amphioxus. This is 

 correct in one sense, but in one sense only. Ontogeny teaches us (and this is borne 

 out by comparative embryology) that the right and left head-cavities of Amphioxus 

 are subdivisions of one anterior body-cavity\ and are not paired structures in the same 

 sense in which the collar-cavities are paired. The praeoral pit therefore should not be 

 defined as the left head-cavity, but as the sinistral portion of the head-cavity which 

 acquires independence and an opening to the exterior. 



1 Which has been called " protomere " by Masterman. 



W. III. 



43 



