578 COMPARATIVE ANATOMY CHAP. 



A true circular muscle layer is nowhere else developed. Such a 

 layer is, however, functionally replaced by pseudo-circular muscle 

 fibres which run on the inner side of the longitudinal musculature, 

 but which in reality do not form a closed ring. 



In Scliizocardium, the bundles of these pseudo-circular muscle fibres run on 

 each side from the dorsal edge of the mediodorsal mesentery to the dorsal edge of 

 the ventral mesentery. Similar bundles arise near the ventral edge of the ventral 

 mesentery, and break up into fibres on the lateral walls of the body, ascending 

 along the inner side of the longitudinal musculature, traversing it, and becoming 

 attached to the limiting membrane of the body. In Glandiceps, this latter system 

 is repeated (but of course reversed) on the dorsal side. 



Balanoglossus has neither the outer true, nor the inner pseudo-, circular mus- 

 culature. 



Radial muscle fibres connect the limiting or basal membrane of 

 the body epithelium with the limiting membrane of the intestine 

 throughout the whole coelom. In the genital folds, these fibres are 

 stretched between opposite points of the integument. In the region 

 of the lateral mesenteries, similar fibres stretch between these and the 

 integument. 



VII. The "Heart Vesicle" (Figs. 456, 21, p. 566; 464, 11, p. 583). 



This is one of the names l suggested for a small closed sac which 

 lies upon the proboscidal divertieulum of the intestine in the basal 

 part of the proboscis. Its ventral wall bends down somewhat over 

 the divertieulum to right and left, and it is separated from the latter 

 by a small blood sinus. Posteriorly, towards the neck of the pro- 

 boscis, the "heart vesicle" is drawn out to a small tip, which is 

 traversed by fibres, most probably muscular, chiefly in a transverse 

 direction, while the rest of the vesicle contains a fluid as clear as 

 water. The median part of the posterior and dorsal wall is in con- 

 tact with the body epithelium of the neck of the proboscis. 



The ventral wall is formed of a single layer of transverse muscle 

 fibres and pear-shaped cells, while the rest of the wall is represented 

 by a plate epithelium. The existence of a closed circular muscula- 

 ture has not yet been demonstrated. 



The " heart vesicle " in Xchiwdrilit/in (and to a lesser degree in Glandiceps also) 

 is ] iroduced anteriorly into two large symmetrically arranged tips, the auricles. 

 From the posterior tip of the " heart vesicle " two bundles of muscle fibres arise which 

 pass anteriorly into these auricles, each one giving off fibres, one after the other, to 

 the wall of the auricle it enters. 



It must be emphasised that the "heart vesicle" does not belong to the blood 

 vascular system, and does not communicate with it, but is merely in contact with 

 part of that system. If, therefore, tin- vesicle propels the blood, this can only 



1 Morgan suggests "Proboscis vesicle." TK. 



