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 ac- 

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

 Hemichorda, 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 Verte- 

 brata 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 Hemichorda (except Cephalodiscus), 

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

 tions 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, Am- 

 phioxus, tunicates, many fishes), though something 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 of Balanoglossus and the whole 

 central nervous system (ganglion) of Cephalodiscus and Rhabdopleura, 

 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. 624, 630). 



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

 obscure. The coelom of the Hemichorda and the Cephalochorda 

 arises, as in the Echinodermata (p. 631), by pouches of the arch- 

 enteron forming three segments, of which the anterior (the proboscis 

 cavity or head cavity) is at least in its beginning median and com- 



