368 



TUNICATA. 



and this depression gives rise to the true cloacal cavity, i.e., to the 

 unpaired part which connects the two peribranchial cavities and 

 which is thus lined with ectoderm. 



The terminal portion of the intestine which had, at an earlier 

 stage, become connected with the left cloacal vesicle (see above, 

 p. 365) now opens into the common peribranchial cavity. 



The heart, the pericardium and the epicardium. We owe to 

 Seeliger the proof that the heart and the pericardium of the 

 Ascidians are entodermal structures, the first rudiment of which can 

 be recognised in the form of an outgrowth from the pharynx arising 

 between the posterior end of the endostyle and the entrance to the 

 oesophagus. A vesicle which becomes abstricted from this caecum 



ot Ji f„ e f 



m 



/ i \ \- i ■'■.-■ < ■ 



es ep> p, 



FlG. 170. — Lett lateral aspect of a Clavelina embryo (after Seeliger). au, eye; ch, 

 -chorda ; >\ exhalent orifice ; ed, rectum ; ep, epicardial outgrowth ; es, endostyle ; 

 /', infolding of the body-surface in anticipation of the rotation that takes place after 

 fixation; /</. ciliated pit; //. adhering papillae; i, inhalent orifice ; ks, gill-slits; 

 ///, stomach; mz, muscle-cells of the tail; oe, oesophagus; "/. auditory organ; />, 

 peribranchial sac ; /»'. pericardium ; s, larval tail ; sb, sensory vesicle. 



(Fig. 170, jic) is the common rudiment of the pericardium and the 

 heart. The remainder of the caecum (ep) has been named by van 

 Beneden and Julin the epicardnvm* These authors were able 

 essentially to confirm the statements of Seeliger, although they 

 repeatedly differ from him in points of detail. They also recognised 

 the significance of the epicardium in connection with budding, and 

 the originally double rudiment of these structures. 



The first rudiment of these organs was observed in the form of two 

 solid cell-strands which run side by side in close contiguity to the 



*[Damas (No. IX.) fluids that in Ciona the epicardium has a paired origin 

 and opens by two distinct orifices into the pharynx. — Ed.] 



