312 MANUAL OF ZOOLOGY SECT. 



exterior by a pair of collar pores ciliated tubes leading into the first 

 gill-slit or first gill-pouch. 



On the dorsal surface of the anterior part of the trunk is a double row 

 of small slits the gill-slits (Fig. 182, br.} each row situated in a 

 longitudinal furrow ; these slits increase in number throughout life. The 

 coelome of the trunk is divided into two lateral closed cavities by a 

 vertical partition (dorsal and ventral mesenteries). 



The mouth (Fig. 183, mo, ) is situated ventrally at the base of the 

 proboscis, within the collar. Into the dorsal half of the anterior portion 

 of the alimentary canal open the internal gill apertures. 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 caca (Fig. 182, 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. 183, 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 nolochord of the typical 

 Chordata. 



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 is situated 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 Tornaria, which is 

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



