CRYPTOCHITON STELLERI MIDD. 21 7 



character are distributed to a greater or less extent throughout 

 the entire foot and mantle but sooner or later they make their 

 way to the main accumulations by means of amoeboid movements 

 and possibly through the agency of the blood stream. In the 

 young of several species of chitons (less than I mm. in length) 

 these concrement bearing cells may be seen to originate from the 

 mesenchyme elements that also form the connective tissue, blood 

 and apparently some of the muscle elements. The concrements 

 arise in the form of one or two small refringent granules in each 

 cell and gradually increasing in size and usually in number sooner 

 or later almost completely fill the cell which during this period 

 usually takes up a position near some blood sinus. In the 

 description of the circulatory system attention is called to the 

 fact that a large blood sinus (lateral sinus) with numerous branches 

 penetrates these " Granulazellen " but the relation of the two 

 remains uncertain. Sections not infrequently give the impression 

 that these cells passing into the sinus disintegrate after which the 

 resulting products may be taken up by the kidney but there is 

 no definite assurance that such is the case. 



There are only two papers in which a serious attempt has been 

 made to trace the circulation of the blood in the chitons. The 

 first by Middendorff is very incomplete and in many respects in- 

 correct ; while the second by Plate is much more detailed yet in 

 certain fundamental particulars, it is not in accord with the results 

 set forth in the following paragraphs. 



As we know from the work of Middendorff the heart in this 

 species is essentially the same as in other chitons consisting as it 

 does of a median ventricle and the lateral auricles. This author 

 however made the mistake of claiming that these latter chambers 

 end blindly behind (for they are united in the usual fashion) and 

 that the ventricle gives off numerous small branches in the median 

 ventral line to the rectum and laterally to the mantle edge and 

 last valve of the shell. The supposed openings in this region are 

 merely the depressions marking the attachment of delicate mus- 

 cle fibers (trabeculae carneae) that span the cavity of the ventricle. 

 In the auricles the same depressions appear but as injections 

 clearly show the only blood, besides that from the efferent bran- 

 chial sinus, that enters these chambers comes from the mantle at 



