ALIMENTARY AND RESPIRATORY SYSTEMS. 399 



the body. Its outer surface is covered by a single layer of 

 ectoderm cells, which secrete the test. Beneath these there 

 lies a gelatinous matrix containing numerous connective 

 tissue cells, blood-carrying spaces, muscle cells forming 

 slender fibres, and so on. The point of importance is that 

 whereas the true ccelom is more or less distinctly visible 

 in the embryo, it is represented in the adult only by these 

 lacunar spaces in the mantle. In other words, in the adult 

 Ascidian, as in Crustacea, the body cavity is haemoccelic. 

 The apparent body cavity of the Ascidian — the space be- 

 tween gut and body-wall — is, as we shall see, lined through- 

 out by ectoderm. 



The muscular system is not well developed. The muscle 

 cells are much elongated and unstriped ; they are aggregated 

 into fibres of varying thickness, which form an irregular 

 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. — On account of 

 the special peculiarities of Ascidia, it is convenient to alter 

 slightly our usual order, and consider these systems next. 



The branchial aperture or mouth opens into a short 

 branchial siphon, separated from the branchial sac itself by 

 a sphincter muscle, whose posterior border is furnished with 

 numerous simple elongated tentacles. In the living animal 

 the tentacles form a sort of network over the opening of the 

 branchial sac. This sac is morphologically the pharynx s 

 and extends almost to the posterior end of the body. It is 

 separated from 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 with the exterior by the 

 atrial opening, and with the branchial sac by innumerable 

 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 large 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 (see Figs. 173 

 and 1 74). The absorbing surface of the intestine is increased 

 by a marked infolding, corresponding to the typhlosole of 



