1888.] between the Body-cavity and the Vascular System. Div 
probably an example of “precocious segregation.” The sexual cells 
arise from the mesoblasts—the segment cells of Whitmann—which, 
arising posteriorly, multiply and pass forward till a heap of them 
is formed laterally in each somite. One pair of these form the 
ovary and seven pairs become testes. According to Nusbaum the 
tunica of the generative glands is formed at the expense of the 
mesoderm. This doubtless buds off corpuscles, just as it does into 
the sinus, and thus forms the colourless corpuscles which Bourne 
found in the fluid surrounding the true ovary. Nusbaum traces 
the oviduct and vas deferens back to nephridia. 
I have attempted so far to shew firstly that there is no doubt 
that the old statements with regard to the blood system of Leeches 
being in communication with the sinus system is true, and secondly 
that the sinus system is coelomic in nature. So that with regard 
to the group Hirudinea, the vascular system is undoubtedly in 
communication with the coelom. 
Let us now turn to the Nemertines, the second group of 
animals mentioned by Sedgwick as forming an exception to the 
rule that the blood system is independent of the coelom. 
The nephridial system of these animals is not so definite in its 
arrangement as amongst the Hirudinea. Oudemans* has examined 
it in a great number of forms, and I have to some extent been 
able to confirm his observations. In his summary at the end of 
his paper he states, “the nephridial system of the Nemertea 
consists of one or more canals, directly communicating, or not, with 
the vascular system, provided, or not, with cilia, and communicating 
with the exterior by means of excretory ducts.” 
But when we come to consider the nature of these spaces 
which contain blood, and in which the internal end of the 
nephridium is sometimes situated, we shall see that they differ 
considerably in their fundamental origin from the sinus system 
of the Hirudinea. 
In his valuable work on the embryology of Lineus obscurus, 
Hubrecht? points out that the blood vascular system together 
with the proboscidian cavity represents the last remnants of the 
archicoel or segmentation cavity. Hubrecht has proposed the 
name archi-coelom for this system of spaces, and in which, as is 
stated above, the inner ends of the nephridia sometimes lie. 
I have already drawn attention® in a previous paper to the 
fact that the cavity of the heart in the embryo Lamprey is 
1 A. C. Oudemans, ‘The Circulatory and Nephridial Apparatus of the Ne- 
mertea.” Q. J. UW. S. 1885. Supplement. 
2 A. A, W. Hubrecht, ‘Contributions to the Embryology of Nemertea.” 
Quarterly Journal of Microscopical Science, Vol. xxv1. p. 417. 
3 «©On some points in the Development of Petromyzon Fluviatilis.” Q, J, M, S. 
Vol. xxv. p. 325, 
