16 PHYLUM TUNIC ATA (UROCHORDA). 



but extends as a single tube into the budding stolon. The 

 identification of this pharyngeal diverticulum with the coelom 

 rests upon the assumption that the pericardium is coelomi?, 

 and upon its structure and developmental relations to the peri- 

 cardium and pharynx in the Clavelinidae, Polyclinidae, and 

 Distomidae. In Ciona an epicardium is present. It has the 

 form of a perivisceral cavity in relation with the digestive 

 viscera and communicates with the pharynx by two openings 

 placed one on each side of the retropharyngeal groove * (p. 9). 

 It is indeed a completely double structure and develops as such 

 from the hind end of the pharynx. It appears that in Ciona 

 the pericardium does not develop from this cavity f as it does 

 in Clavelinidae and some other synascidians. So far as is known 

 Ciona is the only Tunicate with anything corresponding to the 

 coelomic body-cavity of other groups, and it is apparently the 

 only monascidian in which the epicardium is developed. It is 

 possible however that with the extension of our knowledge both 

 these tentative statements may be shown to be erroneous. 



Vascular System. There appears to be a certain number of 

 main blood channels, but the greater part of the circulation takes 

 place in irregular sinus-like spaces in the mesoderm which are 

 said to be without definite walls. The heart is a simple sac or 

 tube formed as above described by infolding of the wall of one 

 side of the pericardium. In Appendicidariae (see p. 64) there is 

 hardly any infolding, and the heart is little more than a contractile 

 membrane. In other forms the infolding is considerable, and 

 the opening of invagination is closed, except at the two ends 

 where it remains open, either by union of its lips or by 

 the closely apposed epicardium. The heart, like the vascular 

 channels of the body, appears to be without endothelial 

 lining, and its walls contain cross-striped muscular fibres. It 

 varies considerably in position ; it is generally placed near 

 the stomach not far from the hind end of the endostyle. In the 

 Polyclinidae it is in the postabdomen. In Ciona it is V-shaptd 

 and lies in the intestinal loop somewhat to the right of the 

 stomach (Fig. 3). 



Each end of the heart is continuous with a blood-channel. 

 The one of these extends on the ventral side of the pharynx 



* NV\v,t.-ud, Q.J.M.S. 3r>. IH't:}. p. 11!). 



t De Selys-Long^huinps. Arch. liiol., 17, I'.tol. p. 499. 



