The anatomy of Lottia gigantea Gray. 31 



is, however, of sufficient thickness to be seen in gross dissection, if 

 sufficient care be taken. 



The blood, on reaching the mantle, courses through a rather in- 

 tricate system on the lower surface, as shown in Fig. 19, and is 

 distributed to the many lamellae. Around the free edge of each of 

 these runs a vein, which proceeds thence to the edge of the mantle 

 after dividing and subdividing many times. These minute vessels are 

 found close under the epithelium, which here consists of rather low 

 cylindrical cells. From the edge of each lamella or lappet very many 

 tiny veinlets pass dorsad on either surface just beneath the low epi- 

 thelium, whence the blood finds its way into numerous venous spaces 

 on the lower surface of the mantle. Thence it either courses toward 

 the edge of the mantle, some of it entering the circumpallial sinus by 

 minute openings in the floor, the rest probably following the other 

 surface blood to the mantle edge, or it passes almost immediately 

 upward through rather indefinite tiny sinuses and enters the arterial 

 spaces (A.Sp) through small and inconspicuous openings. The blood 

 which circulates in the border of the mantle through the fine veinlets, 

 is carried to the large circumpallial sinus by dorsally situated sinuses, 

 some of which open into the main vessel on the outer side, others on 

 the dorso-lateral. Between the circumpallial sinus (which conveys the 

 purified blood to the heart) and the inner edge of the mantle there 

 are very definite spaces, situated above those veins which carry 

 blood to the mantle from the kidney sinus. These arterial sinuses 

 are quite distinct from the veins, which when injected can be seen 

 below them, if one removes the dorsal) wall of the mantle. When 

 this is done it is easy to demonstrate that no direct connection, un- 

 less of the very minutest sort, exists between the vessels below and 

 the sinuses above. Such a connection exists, however, with the capil- 

 lary spaces of the lappets. These sinuses communicate with the cir- 

 cumpallial vessel, by frequent openings. If the reader will imagine 

 the dorsal and ventral sheets of muscle which cross the mantle from 

 inner to outer edge, as being separated by a definite cavity extending 

 from the circumpallial vein to the inner edge of the mantle, and if 

 he will further imagine these two sheets bound together by frequent 

 dorco-ventral trabeculae of muscle and connective tissue, he will gain 

 an idea of the structure of the lacunae. 



In conclusion I might repeat by way of emphasis that venous 

 blood reaches the circumpallial sinus only by very fine vessels, after 



