462 



SANGUIFEROUS SYSTEM. 



mits it through a bulbus arteriosus to a great anterior, (135. 

 e.) and posterior (135. d.) aorta as in conchifera. These two 



FIG. 135. 



cavities are compactly enveloped in a tough pericardial cover- 

 ing, (135. a.) their parietes are strengthened by numerous 

 crossing fasciculi of muscular fibres, and their apertures are 

 well protected by valvular folds. From the varying position 

 of the respiratory organs, the heart is sometimes placed on 

 the right side of the body, sometimes on the posterior or the 

 anterior part of the back, but most frequently towards the 

 left side, and it has commonly a pyramidal or a conical form, 

 with its base, formed by the capacious auricle, directed 

 towards the respiratory organs, and its apex, formed by a 

 strong and angular ventricle, like that of fishes, and a dilat- 

 able bulbus arteriosus, directed towards the systemic aorta. 

 The auricle is divided into two parts in the halyotis^ the 

 Jissurella, and the emarginula, as in the conchiferous animals, 

 and the ventricle of the heart is also perforated by the rectum 

 in these genera, as in the former class. In the. halyotis the 

 arterialised blood returning from two long pectinated bran- 

 chiae is conveyed by two corresponding veins to two length- 

 ened auricles placed on the left side of the body, and much 

 resembling those of most conchifera. From these cavities 

 it is poured on each side into a lengthened fusiform single 

 ventricle placed between them, provided with strong muscular 

 parietes and fleshy columns, and completely embracing the 

 intestine which passes through its cavity. The ventricle 

 indeed, appears externally as only an enlarged or thickened 

 portion of the intestine, to the sides of which the auricles are 



