502 UROCHORDA 



covered by a single layer of ectoderm cells, which secrete 

 the test. Beneath these there lies a gelatinous matrix con- 

 taining numerous connective tissue cells, blood-carrying 

 spaces, muscle cells forming slender fibres, and so on. 



A true coelom has been described in some embryos, but 

 it is afterwards almost suppressed, being represented at 

 most by the pericardium and small lacunar spaces. The 

 apparent body cavity of the Ascidian — the space between 

 gut and body wall — is, as we shall see, lined throughout by 

 ectoderm. 



The muscular system is not w^ell developed. The muscle 

 cells are much elongated and unstriped ; they are aggre- 

 gated into fibres of varying thickness, which form an ir- 

 regular network on the right side of the body, while they 

 are virtually absent on the left. Special sets of fibres form 

 sphincters round the apertures. 



Alimentary and respiratory systems. — The mouth 

 opens into a short stomodasum, separated from the branchial 

 sac itself by a sphincter muscle, whose posterior border 

 is furnished with numerous simple elongated tentacles. 

 Behind this lies a ciliated peripharyngeal groove. In the 

 living animal the tentacles form a sort of sieve over the 

 opening of the branchial sac. This sac is morphologically 

 the pharynx, and extends almost to the posterior end of the 

 body. It is separated frorii the mantle by a space whose 

 dimensions vary greatly in the different regions of the body. 

 This space is the peribranchial chamber, which is formed 

 from the ectoderm, and communicates w'ith the exterior by 

 the atrial opening, and with the branchial sac by innumer- 

 able slits. The remainder of the alimentary canal lies on 

 the left side of the body, between pharynx and mantle, and 

 consists of a short oesophagus leading from the pharynx to 

 the fusiform stomach, and of an intestine which describes 

 an S-shaped curve, and then crosses the atrial chamber, to 

 end in an anus lying beneath the exhalant opening. The 

 absorbing surface of the intestine is increased by a marked 

 infolding, corresponding to the typhlosole of the earthworm. 

 A mass of tubules connected by a duct with the cavity of 

 the stomach is possibly a digestive gland. 



The structure of the pharynx is exceedingly complex, for 

 it has a double function — respiratory and nutritive. More- 



