CRYPTOCHITON STELLERI MIDD. 2 19 



blood from the mantle passes into both the branchial artery and 

 the branchial vein. In Cryptochiton the vessels from the mantle 

 are of comparatively large size and may be followed without 

 difficulty into the lateral sinus. Injections of the branchial 

 artery and the branchial vein give no indication that they receive 

 any blood directly from the mantle. 



The marginal vessel (Randgefass) imbedded in the mantle to 

 the outside of the mantle furrow, a position corresponding to the 

 lateral fold (Lateralfalte) of other chitons, is but a system of 

 irregular sinuses supplied by vessels from the intersegmental 

 artery. The blood returning from it passes, together with other 

 blood from the mantle, into the lateral sinus. 



In Cryptochiton the course of the blood leaving the head 

 cavity is considerably different from that in any other chiton 

 hitherto described. According to several investigators the blood 

 leaves the head sinus by the following vessels : the visceral 

 artery, the lateral sinus of the foot, vessels passing into the 

 snout (Mundscheibe) and by means of canals surrounding the 

 pedal and pallial nerves. In Cryptochiton no blood makes its 

 direct exit by either the pedal or pallial neural sinuses and the 

 course of the blood passing into the snout is considerably dif- 

 ferent from that described by Plate for Acanthopleura cchinata, 

 and furthermore there is a pair of pallial arteries. 



The head cavity in the chitons is a comparatively narrow 

 space surrounding the buccal mass and separated from the vis- 

 ceral cavity by a septum. Among the more important arteries 

 leading out from it are two of large size which spring from the 

 postero-lateral borders of the snout (Fig. 3). Almost imme- 

 diately each divides, the inner branch, the pedal sinus, passing 

 into the foot, the outer canal, the pallial artery, following along 

 the mantle cavity in close proximity to the pallial or lateral nerve. 

 The pallial sinus shortly after its origin gives off a branch which 

 ramifies throughout the tissue of the proboscis supplying with 

 its fellow all of the region behind the mouth. It is also in com- 

 munication with an extensive sinus encircling the snout as will 

 be described presently. By means of injections each pallial sinus 

 may be followed into the region of the reproductive opening 

 where, after having gradually diminished in size, it vanishes com- 



