PHYLUM CHORDATA 



313 



of the alimentary canal open the internal gill-openings. The gill- 

 pouches are supported by a chitinoid skeleton consisting of a number 

 of separate parts. 



The posterior part of the alimentary canal is a nearly straight tube 

 with, in its middle part, paired hepatic cosca (Fig. 195, hep), which 

 bulge outwards in a series of external prominences. Posteriorly it 

 terminates in an anal aperture situated at the posterior extremity of the 

 body. Throughout its length it lies between the dorsal and ventral 

 divisions of the vertical partition, which act as mesenteries. 



In front the dorsal wall of this anterior portion of the alimentary 

 canal gives off a diverticulum (Fig. 196, div), the lumen of which 

 extends nearly to the anterior end. This diverticulum consists of 

 epithelium with gland cells and of a sort of retiform connective tissue; 

 it is supposed to be homologous with the notochord of the typical 

 Chorclata. 



There is a blood-vascular system with dorsal {dors, v) and ventral 

 {vent, v) longitudinal trunks. The nervous system consists of dorsal 

 {dors. n) and ventral strands {vent, n), which extend throughout the 

 length of the body. The part of the dorsal cord which lies in the collar 

 lies deeper than the rest, and contains a canal or a number of spaces. 

 Between the collar and the trunk the dorsal and ventral strands are 

 connected by a ring-like thickening. There are no organs of special 

 sense. 



The sexes are separate; the ovaries and testes are saccular organs 

 arranged in a double row along the branchial region of the trunk and 

 further back ; they open on the exterior by a series of pores. 



The course of the development differs in different species. In some 

 it is comparatively direct; in others there is a metamorphosis. In the 

 latter case the embryo assumes a larval form termed Toniaria, which 

 is somewhat like an Echinoderm larva, with a pair of ciliated bands, 

 one of which is considered pra;-oral, and the other post-oral, and an 

 independent circlet of strong cilia at the posterior end. 



Usually associated with Balanoglossus are two aberrant animals — 

 Cephalodiscus and Rkabdopleura — formerly regarded as Polyozoa. 

 These both resemble Balanoglossus in having the body divided into 

 three parts or regions — a proboscis, with a proboscis cavity, a collar 

 with a collar-cavity communicating with the exterior by a pair of collar- 

 pores, and a trunk with two distinct lateral cavities; and in the presence 

 of a structure resembling a notochord with the same relations to the 



