358 ALFRED GIBBS BOURNE. 
go, it is always built upon the same general type as in 
M. grandis. 
Nerve-cord.—This presents no marked variations, 
Generative Organs.—It is with regard to these organs 
that the greatest variations are usually looked for, but there is 
in this genus, so far as it is known to me, the most striking 
anatomical, and indeed histological, uniformity in this matter. 
The testes may vary in the position of their attachment to the 
wall of the sperm-sac. The inner wall of the prostate may or 
may not present a layer of muscle between the gland-cells and 
the coelomic epithelium; in the former case these organs 
present on opening the worm a shining appearance, and in the 
latter their glandular nature is very apparent. I express the 
latter condition by saying “ prostates glandular in appearance.” 
I have not examined them all histologically, and there may be 
in some species a few muscle-fibres present, even when I have 
called them “ glandular ;”’ but anyone who has compared the 
“slandular” with the “non-glandular” appearance cannot 
fail to distinguish between them, and I have never found 
individual variation in the matter. 
The precise point of penetration of the sperm-duct into the 
prostate varies in different species. The prostates vary in 
shape to a limited extent in different species, but they also vary 
according as the glands are full or empty of secretion. 
The spermathece and sperm-sacs vary in shape in different 
species, but they also vary according as they are full or empty, 
a pyriform spermatheca becomes almost globular when quite 
full, and an oval sperm-sac becomes to a slight extent broader 
along its shortest axes. 
The most valuable character is to be found in the copulatory 
pouch ; this is in some species small and completely embedded 
in the body-wall, and at first sight would be overlooked ; in 
others it is large, but not bilobed; in others, again, it is bilobed. 
I have never found individual variations in respect of this 
matter except in the ophidioid group of species, where I 
think hybrids occur ; this matter is dealt with elsewhere in this 
paper, It is not easy always to see, but I believe that the 
