22O 



HAROLD HEATH. 



pletely. Delicate branches supply the tissue of the immediate 

 region, some going to the lateral nerve another portion to the 

 tissue of the lateral space and it is possible that another quantity 

 though very small may enter the adjacent regions of the mantle. 

 Middendorff (pp. 69, 70) speaks incorrectly of two pairs of 

 vessels which arise from the head cavity and passing in close 

 proximity to the mantle furrow open into the sinus transver- 

 sus (arcus arteriosus). In Cryptochiton the only vessel which 



Fig. 3. Diagram illustrating the circulation of snout and certain vessels leaving the 

 head cavity in C. stelleri. h, head cavity; /, lateral nerve sinus; Is, lateral sinus; 

 m, median proboscis protractor; mf, marginal proboscis protractor; /, pedal sinus; 

 pn, pedal nerve sinus ; s, pallial sinus : vs, visceral cavity ; nervous system, black. 



holds such a position posteriorly is the lateral sinus and this as 

 is well known has no connection whatever with the head cavity, 

 and on the other hand the pallial artery connects with the head 

 cavity but does not communicate with the transverse sinus. 



Heretofore it has been held that the sinuses surrounding the 

 pallial cords conduct a considerable amount of blood from the 

 head cavity into the mantle but in not less than 50 specimens of 

 Cryptochiton injections from the head sinus failed to penetrate 



