CHAPTER XIX 



THE PROTOCHORDATA 



Most of the members of the phylum Chordata do not come within 

 the scope of this book. But, though a position in that phylum is 

 accorded by all authorities to the Tunicata and by many also to the 

 Enteropneusta, it is often convenient to treat of both these groups 

 with the other invertebrate animals, and we shall take that course. It 

 will be well, however, first to indicate what are the features which the 

 groups in question share with the other chordate animals — the 

 Vertebrata proper, and the Cephalochorda (Amphioxus) , which are 

 usually studied with the Vertebrata. The Chordata are bilaterally 

 symmetrical, coelomate metazoa which have in common certain 

 fundamental features stated in the following paragraphs. 



(i) With the single exception of the minute, sessile Rhabdopleura , 

 every member of the group possesses, at least in its early stages, 

 lateral perforations from the pharynx to the exterior which are known 

 as gill clefts or, by the name which is applied to those of them which 

 in vertebrata do not bear gills, as visceral clefts. Moreover, the gill 

 clefts of the Cephalochorda, the Enteropneusta (except Cephalodiscus)y 

 and many tunicates, have the further resemblance that the per- 

 forations which originate them are subsequently divided by 

 tongue bars, so that each gives rise to two of the definitive clefts. It 

 is probable that the original function of the "gill" clefts was the 

 filtering off of food from water taken in through the mouth. This 

 function they still retain in the lower members of the phylum 

 (Balanoglossus, Amphioxus, tunicates, many fishes), though some- 

 thing of a respiratory function is perhaps always superadded to it. 



(2) In all the Chordata the central nervous system (a) arises from a 

 median dorsal strip of ectodermal epithelium from which it never 

 parts, (b) is, except in the trunk cord oi Balanoglossus and the whole 

 central nervous system (ganglion) of Cephalodiscus and Rhabdopleuray 

 removed from the surface of the body, by invagination or by over- 

 growth of the epithelium at its sides, so as to form a tube, lined by its 

 persistent epithelium. These features of the nervous system of the 

 Chordata have analogies in that of the Echinodermata (pp. 547, 554). 



(3) The common features of the coelom of the Chordata are more 

 obscure. The coelom of the Enteropneusta and the Cephalochorda 

 arises, as in the Echinodermata (p. 5 5 5), by pouches of the archenteron 

 forming three segments, of which the anterior {the proboscis cavity or 

 head cavity) is at least in its beginning median and communicates 



