474 



LECTURE XX. 



layers, — the external circular, the internal longitudinal. The fibres 

 or fasciculi of the outer layer are smaller than those of the inner one, 

 and less regularly disposed. They describe regular circles around the 

 processes leading to the orifices of the shell. Other fibres of the outer 

 layer pass transversely from one tube to the other. The longitudinal 

 fasciculi radiate from the two orifices, and decussate each other, 

 winding round the bottom of the sac. Deeper, again, than this layer 

 there is a sphincter surrounding the base of each tube, or orifice, from 

 which a third more delicate layer of longitudinal fibres is given off". 



Of the two more or less protuberant and stellate apertures in the 

 outer tunic, one (b) leads directly into the muscular sac, the other (a) 

 into a wide vascular branchial sac contained in the muscular one. 

 The entry to the branchial sac is defended by a circle of short ten- 

 tacles (Jig.l79,f). The sac is dissected away in this figure. The inner 

 surface of the sac is marked by parallel 

 and equidistant transverse lines, the 

 interspaces of which are divided into a 

 series of narrow vertical, perforated, 

 and richly ciliated compartments ; two 

 opposite narrow longitudinal tracts are 

 entire (fgs. 180, 184,/,/). A groove 

 along that which traverses the larger 

 curvature of the sac leads to the 

 mouth — an orifice {Jig. 179, a) at fhe 

 bottom of the branchial sac, which 

 conducts, by a short oesophageal canal, 

 to the stomach [b) ; this is an oblong 

 cavity, with longitudinal folds. The 

 intestine is disposed in a sigmoid 

 flexure, adheres to the outside of the 

 branchial and the inside of the mus- 

 cular sac, and terminates by a fim- 

 briated anal aperture (c) near the base 

 of the second orifice of the tunic (d). 



The liver consists of blind follicles, Cynthia canopus. 



produced into tubes which anastomose, surrounding more or less of 

 the intestine, as by a net-work, and ultimately, at least in Cynthia 

 tuberculata, communicating with the stomach by a single aperture, 

 from which a groove is continued towards the cardia. 



The heart (Ji) is a simple, elongated, vasiform muscle, inclosed in 

 a pericardium, attached to the branchial sac ; continued at either end 

 into a vessel ; the ramifications of one being expended chiefly upon 

 the respiratory organ ; those of the other upon the tunics of the body, 

 or speedily expanding into sinuses surrounding the viscera. Ac- 



