COELOM EPICARDIUM. 15 



is based does not occur. The vascular space in Tunicates as 

 in other forms is a space in the so-called mesodermal tissues of 

 the body. As it frequently appears before the mesoderm is 

 extensively developed, it seems to lie between ectoderm and 

 endoderm arid to be bounded by these layers, and in this 

 respect has the relations of the segmentation cavity or blasto- 

 coel. But it is not derived from the blastocoel, which closes 

 up by the coming together of the ectoderm and endoderm on 

 the completion of the gastrula invagination. 



Excluding the generative organs, which will be dealt with 

 below, it would appear that the only possible representatives of 

 the coelom are the pericardium and the epicardium. The 

 pericardium is a closed epithelial sac found in all or almost 

 all Tunicates in the neighbourhood of the stomach and not far 

 from the hind end of the endostyle. It is developed in the 

 embryo, in some forms at any rate, as a diverticulum of the 

 epicardium (see below). One side of it, generally the dorsal, is 

 invaginated upon the rest much as a blastosphere is invaginated 

 to form a gastrula. The space enclosed by the invaginated wall 

 and corresponding to the cavity of the gastrula is the heart. 

 As the aperture of invagination never completely closes, the cavity 

 of the heart communicates, generally at its two ends, with the 

 haemocoelic spaces of the body. The contraction of the heart 

 is effected by the invaginated wall which acquires cross-striated 

 contractile fibres on that side of it turned towards its cavity ; 

 the inner side, i.e. the side turned towards the pericardial cavity 

 remaining epithelial. It results from this mode of origin that the 

 heart is without an endothelial lining. 



The epicardium is usually found only in the budding forms. 

 It opens into the pharynx by a median opening just behind the 

 endostyle, or by two openings, one on each side of the middle 

 line between the end of the endostyle and the oesophagus. 

 It passes backwards and extends into the abdomen (Clave- 

 linidae, Polydinidae, Distomidae), where it is closely applied to 

 the dorsal wall of the pericardium, forming indeed, when this 

 is invaginated, the actual dorsal wall of the heart. In these 

 forms it has been shown that the pericardium is actually developed 

 from it, and it always extends into the stolon and gives rise to 

 an important constituent of the buds. In Pyrosoma, and the 

 Thaliacea it has not this close connexion with the pericardium 



