132 EARLY STAGES IN THE 



quently become united below and converted into a single cavity 

 (vide loc. cit.\ Plate XIV, fig. 8 b, //). 



In the Echinoderms we find instances where the body cavity 

 and water-vascular system arise as an outgrowth from the ali- 

 mentary canal, which subsequently becomes constricted off from 

 the latter (Asteroids and Echinoids), together with other instances 

 (Ophiura, Synapta) where the water-vascular system and body 

 cavity are only secondarily formed in a solid mass of mesoblast 

 originally split off from the walls of the alimentary canal. 



These instances shew us how easily a change of this kind 

 may take place, and remove the difficulty of understanding why 

 in vertebrates the body cavity never communicates with the 

 alimentary. 



The last point which I wish to call attention to is the blasto- 

 pore or anus of Rusconi. 



This is the primitive opening by which the alimentary canal 

 communicates with the exterior, or, in other words, the opening 

 of the alimentary involution. It is a distinctly marked structure 

 in Amphioxus and the Batrachians, and is also found in a less 

 well-marked form in the Selachians ; in Birds no trace of it is any 

 longer to be seen. In all those vertebrates in which it is present, 

 it closes up and does not become the anus of the adult. The 

 final anus nevertheless corresponds very closely in position with 

 the anus of Rusconi. Mr Lankester has shewn (Quart. Journ. 

 of Micro. Science for April, 1875) that in invertebrates as well as 

 vertebrates the blastopore almost invariably closes up. It never- 

 theless corresponds as a rule very nearly in position either with 

 the mouth or with the anus. 



If this opening is viewed, as is generally done, as really being 

 the mouth in some cases and the anus in others, it becomes very 

 difficult to believe that the blastopore can in all cases represent 

 the same structure. In a single branch of the animal king- 

 dom it sometimes forms the mouth and sometimes the anus : 

 thus for instance in Lumbricus it is the mouth (according to 

 Kowalevsky), in Palaemon (Bobretzky) the anus. Is it credible 

 that the mouth and anus have become changed, the one for the 

 other ? 



If, on the other hand, we accept the view that the blastopore 

 1 PI. 3 of this edition, fig. 8 /-. />/>. 



