STRUCTURE AND DEVELOPMENT. 433 



of life for the adult. These germinal defects are manifestly cumulative in their 

 results for, as a rule, only the older stages are so modified by them that practically 

 all traces of the initial organs are obliterated. The adult parasitic crustacean 

 with its worm-like body, devoid of metameres and appendages, and with sense 

 organs, nerves, muscles, heart, mouth, and alimentary organs either imperfectly 

 developed, or altogether absent, could not have been recognized as arthropods, 

 with highly specialized ancestors, if it were not for the absolutely conclusive testi- 

 mony of the embryonic and larval stages. 



In the enteropneusta we have a precisely similar condition, except that the 

 particular form of reduction characteristic of these animals is not dependent for 

 its perpetuation upon a parasitic mode of life; and the loss of organic definition 

 extends farther back into the ontogeny, modifying and disguising the embryonic 

 and larval stages, almost as effectually as it does the later ones. Nevertheless, 

 certain characteristic conditions have been retained that demonstrate with reason- 

 able certainty that the enteropneusta are descended from arthropod stock. 



Structure and Development. — The cleavage is total and the resulting 

 blastomeres are of remarkably uniform size. A deep infolding is formed at the 

 caudal end, representing a typical telocoele, or mesentocoele. The telopore soon 

 closes, but opens again as the anus, or the anus forms at the point where the 

 telopore closes. (Fig. 270, A.C.) 



The Gastrula and the Telocoele. — At a very early stage a large portion of the 

 inner tube is constricted off, and a funnel-shaped outgrowth extends from it 

 toward the anterior neural surface, where a median ectodermal infolding is formed 

 that unites with it, putting the inner chamber into communication with the 

 exterior. 



The history of the important events that take place at this point is by no 

 means clear or conclusive. The early opening in the median neural surface of 

 the proboscis has been called the proboscis pore and has been compared with the 

 hydropore of echinoderms, but the difference in their location is apparently 

 irreconcilable, one being on the haemal side and the other on the midneural side. 

 Moreover, the primitive proboscis pore does not appear to be the same thing as 

 either the single unsymmetrical proboscis pore, or the two pores that may be 

 present in the adult. 



A more satisfactory explanation may be given, it seems to me, and is as fol- 

 lows: The anterior section of the inner tube represents the remnants of the gas- 

 trula, and consists of the primitive cephalic mesoderm and endoderm, at first 

 united with the walls of the teloccele, later separating from them as the so-called 

 "proboscis ccelom." (Fig. 270, g.) The point of union with the procephalic lobes 

 represents the remnants of the blastopore, and the coincident ectodermic infolding, 

 the neurostoma, and primitive stomodffium, n.st. The latter closes and loses its 

 connection with the cephalic endoderm, but leaves for some time a faint central 

 depression that marks its original location. (Fig. 296, n.st). The definitive 

 proboscis pores arise later, close to the neurostoma, and open into the proboscis 



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