596 THE INVERTEBRATA 



epithelium of the digestive part of the alimentary canal is ciliated, 

 and a gland ramifies in the wall of the stomach, into which it opens by 

 a duct. The faeces are cast out by the outgoing current from the atrium. 



The stomach and intestine lie in a section of the body known as the 

 abdomen, which is behind (basal to) the region (called the thorax) in 

 which the pharynx and atrium are situated. The viscera just men- 

 tioned are enclosed in a perivisceral space, known as the epicardial 

 cavity, which is of a very peculiar kind, since it is formed by two thin- 

 walled outgrowths from the pharynx, one on each side of the retro- 

 pharyngeal band. Epicardial diverticula of the pharynx are found in 

 many tunicates (p. 599), but it is only in Ciona that they expand and 

 form a perivisceral cavity, applying their walls as a peritoneum to the 

 contained organs. In this cavity lies also the heart, a V-shaped tube 

 placed in the intestinal loop, near the hinder end of the endostyle. 

 The heart has no proper wall but is formed by the folding-in of one 

 side of the tubular pericardium, which on this side is muscular. The 

 other blood vessels also have no walls of their own, but are mere 

 vacuities between various structures. In these respects the Tunicata 

 resemble the Hemichordata (see p. 587). Each end of the heart is 

 continuous with a bloodvessel. The vessel from one end runs forwards 

 under the endostyle and communicates with the blood spaces in the 

 branchial bars: these in turn join a vessel, in the hyperpharyngeal 

 band, which gives off branches to the digestive organs, gonads, and 

 body wall. To these same organs runs the vessel from the opposite 

 end of the heart. The course of the circulation is remarkable. The 

 heart for several beats drives the blood towards one end and then 

 reverses its action. Thus the blood passes, at one time, like that of 

 a fish, through the gills to the rest of the system, and at another in 

 the opposite direction. The plasma is colourless, but contains nucleated 

 corpuscles, some of which are of various colours owing, remarkably 

 enough, to the presence of compounds of vanadium. 



The animal is a hermaphrodite. The ovary lies between the stomach 

 and intestine as a compact mass ; the testis ramifies over the stomach 

 and intestine. The genital ducts run side by side along the rectum and 

 at some distance beyond its end open into the cloaca. The vas de- 

 ferens is the narrower, and has a patch of red excretory cells around 

 its enlarged end, where it opens by a rosette of small pores. Fertili- 

 zation takes place in the water, and the spermatozoa will not unite 

 with ova from the same individual. 



The central nervous system is reduced to a single elongated solid 

 ganglion (brain) on the dorsal side between mouth and atrial opening. 

 From its ends nerves are given off. There are no organs of special 

 sense unless the pigment spots between the lobes of the mouth be 

 functional for the perception of light. 



