HAROLD HEATH. 



ply but as may be seen in living specimens the more central 

 portions bounding the mouth are likewise subject to great disten- 

 sion. This is produced by a system of vessels present also in 

 Ac -antko pleura ccliinata according to Plate though their connec- 

 tions are considerably different. From the extreme anterior and 

 ventral part of the visceral cavity they arise as two relatively very 

 wide canals one on either side of the mid-line. Coursing almost 

 directly forward they enter the proboscis and giving off vessels 

 into the more superficial portions encircle the mouth almost to 

 the mid-line in front. Upon the contraction of some of the more 

 anterior pedal muscles the visceral cavity is decreased in volume 

 and a portion of the blood coursing through spaces in the region 

 of the stomach is forced into the proboscis resulting within a 

 short time in a marked increase in size and roundness of its cen- 

 tral portions. In recently killed specimens pressure on the front 

 part of the foot produces this phenomenon. 



From injections it appears very probable that a considerable 

 portion of the venous blood from the snout, when this is not pro- 

 truded, passes into the visceral cavity by means of these protrac- 

 tor sinuses. The marginal portions of the proboscis and possibly 

 the more central regions also are relieved of a portion, seemingly 

 small, of their blood supply by means of several highly branched 

 vessels emptying into the lateral sinus external to the snout. 



In Acantlioplcnra ccliinata according to Plate the foot is pro- 

 vided with a median sinus which anteriorly connects with two 

 vessels surrounding the mouth and corresponding to the median 

 proboscis protractors of Cryptochiton. Were these latter vessels 

 in Cryptochiton to be separated with a portion of the visceral 

 cavity and fashioned into a median pedal sinus the relations would 

 be essentially the same as in the above-named species. However 

 the course of the blood leaving the median sinus is not by the 

 usual exit from the visceral cavity for Plate states that it passes 

 into the branchial artery. 



The blood enters the foot by the two pedal sinuses, whose rela- 

 tions are already known, and leaves by two routes into the vis- 

 ceral cavity or along the transverse pedal muscles to the kidney. 

 As has been shown the contraction of some of the anterior pedal 

 muscles may drive a quantity of blood into the proboscis, thus 



