ANATOMY AND DEVELOPMENT. 1 91 



the extent that the subneural organ of the Ascidians is 

 frequently spoken of as the hypophysis. 



Circulatory System. 



With regard to the circulatory system the Ascidians 

 differ markedly from Amphioxus in the possession of a 

 well-defined heart which lies in a distinct pericardium. 

 The heart lies ventrally and usually in the neighbourhood 

 of the stomach. (Cf. Fig. 96.) Its wall is muscular, but 

 consists only of a single layer of cells whose deeper portions 

 (i.e. towards the cavity of the heart) are drawn out into 

 striated muscular fibres, while the outer portions of the 

 cells containing the nuclei project into the cavity of the 

 pericardium. 



There is therefore no true endothelial lining to the heart, 

 and the cells which build up its wall offer a most interest- 

 ing example of epithelio-muscular tissue, as was first pointed 

 out by Edouard van Beneden. This type of muscular tis- 

 sue, in which the muscle-fibres occur as basal prolonga- 

 tions of cells which still retain their epithelial character, is 

 found, as is well known, in the case of the body-muscles of 

 the Nematode or thread-worms, and is above all character- 

 istic of the Ccelenterata (Hydroids and Medusae). 



There are no true blood-vessels in Ascidians, but the 

 passages along which the blood percolates are merely 

 lacunae in the connective tissue and musculature of the 

 body and between the viscera. They are not lined by an 

 endothelium, and are more correctly described as. blood- 

 sinuses. They are often irregular in their outline, as shown 

 in the transverse section represented in Fig. 95, but often 

 again they simulate the appearance of true blood-vessels, 

 as in the case of those branches which pass from the 

 mantle into the substance of the test, as well as the tubes 



