372 MR. J. D. MACDONALD ON THE CIRCULATION IN PEGEA. 



rests upon the nervous ganglion (fig. 2, e) ; and the blood-corpuscles may be always seen 

 to pass above, but never beneath the latter. Many vascular trunks radiate from this 

 point ; thus one (fig. 2, a) courses directly backwards in the median line ; from the fore- 

 part two channels {b h) diverge, passing forwards and outwards ; while several others pass 

 outwards and upwards on either side {c), and two in particular ((/ d) from the upper sur- 

 face enter the gill. Now, all these vascular channels, according to then' position and 

 direction, become continuous with the subdivisions of those already described as arising 

 from the heart, without any interposition of lacunsB properly so called, and the scheme of 

 circiilation is thus completed. 



A superficial reticulation of vessels occurs between the muscular bands and the delicate 

 membrane which immediately covers them, while the larger trunks evidently lie between 

 the muscular and branchial tunics. The muscular coat is therefore very frequently per- 

 forated by communicating canals ; so that an arrangement similar to that given in fig. 3, 

 representing a portion of the ventral sinus with its contiguous vessels, is easily demon- 

 strable, and affords a strong indication of a lining to the vascular system, though such has 

 of late been very nearly denied to the Timicata altogether, while it is admitted only in a 

 limited sense in the Ilollusca proper. (See Von Siebold and others.) 



EXPLANATION OF PLATE XXXV. 



Fig. I. Heart and great vessels of Peyea. a, visceral nucleus; b, chaplet of young; c, " eleeoblast ;" 

 d, heart ; e, gill band ; f h g, vessels arising from the anterior extremity of the heart ; h, en- 

 dostyle ; i, great vessel arising from the perivisceral sinus ; k, diverticula ; /, respiratory vessels ; 

 m, mouth with labial folds, &c. 



Fig. 2. Nervous ganglion and radiating sinus, a, ventral sinus ; b, antero-lateral trunks ; c c, lateral 

 truuks ; d d, respiratory vessels ; e, ganglion with its radiating nerves and otolithic sac. 



Fig. 3. Portion of the ventral sinus, connected with the superficial plexus, a, main trunk ; b b, super- 

 ficial plexus ; c, linear-extra- vascular space. 



