MOUTH AND ANUS. 8 



the adult, and its transitory existence is a highly remarkable 

 fact for which no satisfactory explanation has ever been offered. 

 In the Enteropneusta alone is the central nervous system 

 confined to a short portion only of the dorsal surface (so-called 

 collar region), and m them alone does the central canal remain 

 permanently open and never acquire a communication with 

 the enteron. 



We have said that the central nervous system arises on the 

 dorsal surface. Now it is quite clear that this surface corresponds 

 to the ventral surface of other Coelomata, so that it would 

 be convenient to excliange the term dorsal for a term which would 

 include the same surface in all Coelomata. Such a term is 

 afforded by the term neural surface, which implies, and cor- 

 rectly implies, that the central nervous system is developed 

 upon it. Another term, blastoporal, having reference to the 

 position of the embrj^onic blastopore might also be used. In 

 all the Coelomata the blastopore is not only placed on the neural 

 surface of the body, but actually perforates the embryonic 

 rudiment of the central nervous system. This is seen most 

 clearly in tlie embryonic history of the Cephalochorda, the Verte- 

 brata, the Annelida, Arthropoda and Mollusca. In the Entero- 

 pneusta and Echinodermata this relation is masked, and by 

 many morphologists would be held not to occur at all. But that 

 it does exist we are convinced, and is a most important morpho- 

 logical fact appertaining to all Coelomata. Now in some Coelo- 

 mata it has been definitely proved that the mouth and anus 

 of the adult animal are directly derived from the embryonic 

 blastopore, and it becomes a question whether this deriv^ation, 

 though not embryonically manifested in all forms, does not 

 also hold throughout the Coelomata. Believing as we do 

 in the homology of the mouth and anus, at least in the 

 phyla Annelida, Arthropoda and Mollusca, it follows that 

 this relation holds for them. In Peripatus the mouth 

 and anus are not only derived from the elongated blasto- 

 pore by its constriction into two openmgs, but remain 

 throughout life included within the nerve ring derived 

 from the neural rudiments of the embryo.* If in other 



* Sedgwick, " Monograph of the Development of Peripatus capensis," 

 Studies from the Morphological Laboratory of the University of Camhridfje, 

 A, 1889, p. 1. 



