554 H, p. KJERSCHOW AGERSBORG 



is like that of a lymph-node (PI. 34, fig. 59, Bgl). Within this 

 gland (node) are a number of free cells (PI. 34, fig. 56), which 

 vary considerably in size and structure (PI. 34, fig. 58, a, b, c). 

 Some of the cells of the inside of the gland are pseudopodic (a), 

 which is also perhaps the condition of the cells free in the 

 lumen (PI. 34, figs. 56, L, 58, c). The structure of these cells 

 may also be the same, but the cells within the gland remain 

 in a more stable environment during the time of death, and 

 on that account may be less subjected to physical shock than 

 the cells within the lumen of the blood-vessels at the time 

 of the kiUing. In fact, the cells from the lumen (PI. 34, fig. 58, c) 

 show a considerable morphological difference in that they are 

 highly vacuolated. It is known from the study of invertebrate 

 blood that the cells contained in the blood-fluid are exceedingly 

 unstable (Tait and Gunn, 1918). In fact, these men w^ere 

 able to destroy the blood-cells of the circulation of the 

 fresh-water crayfish, Astacus fluviatilis, by injecting 

 India ink into the circulation of the living animal ; the cells 

 explode very easily upon contact with foreign solids. The 

 cells of the circulation of Melibe may not be as easily 

 destroyed or as sensitive as the blood-cells of Astacus. 

 Whether the difference in environment relative to that of the 

 blood-plasma and the node is sufficient to produce this differ- 

 ence in post-mortem structure by the same killing method 

 cannot be determined here. 



Boas (1886) finds for C 1 e o d o r a a c i c u 1 a that the ventricle 

 is constructed of a few, large, short, flattened, perhaps muscle- 

 cells, which touch each other by their edges. Each cell has 

 a nucleus which lies on the outside of the contractile substance, 

 surrounded by a small protoplasmic mass. The contractile 

 substance consists of fine fibrillae, which are visibly trans- 

 versely striped. In M. leonina the cardiac wall does not 

 consist of muscle-cells exclusively, but of large nucleated 

 fibrillated epithelioid cells (PL 34, fig. 57, Cfm), but I cannot 

 at this time tell definitely whether the fibrillae are striped 

 transversely ; it is quite evident, however, that these cells 

 are different from the muscle-cells of other organs and of the 

 body-wall of this animal. 



