216 Mr A. E. Shipley, On the Existence of Communications {May 7, 
direction. The fluid and corpuscles in both vessels and sinuses 
being apparently identical. 
The foregoing facts fully corroborate Bourne’s statements that 
the nephridia open into the sinuses, which in their turn are in 
communication with the blood vessels, and which contain the 
same fluid as the vascular system. With regard to the embryo- 
logical nature of these spaces we are largely indebted to the 
researches of Nusbaum*. He describes in Clepsine the meso- 
blastic bands dividing into 33 somites. Hach of these somites 
acquires a cavity which gradually increases in size. The walls of 
this cavity on the upper side, towards the endoderm, become only 
one cell thick, they form the splanchnopleure. The opposite wall, 
the somatopleure, that next the ectoderm, is however several cells 
thick. 
The anterior wall of each somite fuses with the posterior wall 
of the preceding somite, and thus septa, comparable to those of the 
higher worms, are formed, and persist for a short time in embryonic 
life. Soon, however, the somites fuse with one another, and their 
cavities become continuous. Then the walls of the two lateral 
cavities which are thus formed, and at first are only im the ventral 
face of the embryo, commence to grow round the endoderm. Part 
of the tissue forming the septa persists as the dorso-ventral muscles. 
The spaces on each side, growing dorsally and ventrally, fuse, and, 
by the arrangement of the dorso-ventral muscles two longitudinal 
septa are formed which divide the common space into a dorsal, 
ventral, and two lateral sinuses. These are the blood sinuses, 
which by the development of the connective tissue and muscles 
become relatively much smaller in the adult than in the embryo. 
Nusbaum further describes and figures the development of the 
dorsal and ventral vessel, both of which apparently arise as a solid 
cord of cells, proliferated from the splanchnic layer of the meso- 
derm, in the middle dorsal and ventral line. They subsequently 
acquire a lumen, and, separating off, lie in their respective sinuses. 
The same author, in describing the development of the nephridia, 
points out that in the young embryo they appear in every somite, 
even in those which form the posterior sucker, where they subse- 
quently abort. 
Thus with regard to the origin of the space and the opening 
into it of the nephridia, the sinuses of the Hirudinea are truly 
coelomic, the embryological researches of Nusbaum confirming in 
a most striking way the predictions of Bourne. 
If we turn to the third characteristic of a coelom, that “its 
lining gives rise to the generative products,” the evidence is not 
quite so satisfactory. The origin of the reproductive cells is 
1 Archives Slaves de Biologie, Vol. 1, pp. 320 and 539, 
