452 W. BLAXLAND BENHAM. 
this ridge sends down a branch (2) crossing the convex surface 
and ending abruptly, like the main ridge, close to the body- 
wall. This ridge throughout its extent is traversed by a very 
narrow furrow. 
The histological structure of this peculiar organ, so far as it 
can be determined on my specimens, is fairly simple; it is a 
solid mass of muscle, covered by an epithelium of a single layer 
of cells (fig. 17), as will be more fully described below. 
Presumably this pair of organs serves to hold two animals 
together during copulation, the organs of each clasping the sides 
of the other, somewhat in the same manner, no doubt, as the 
peculiar “ penial appendages,” or, as I would call them, claspers 
of Siphonogaster (Alma); the chief differences between the 
two organs being the presence of chet and the absence of 
any power of withdrawal of the organ into the body in the 
case of the latter. 
We know little of the mode of copulation even in our native 
earthworms, but we can distinguish at least four kinds of 
apparatus for holding the two worms together :— 
1. The penis-like terminal duct of the spermiducal gland of 
Pericheta, Acanthodrilus, and other worms, which appears 
tobe capable of pleurecbolic eversion, and is presumably received 
by the copulatory sac, a portion of the spermatheca; tosuch an 
apparatus the term “ penis ” appears applicable. 
2. “ Suckers,’ such as I have described in Microcheta 
papillata; and under this head we must include probably 
the terminal “atrium” of the sperm-duct of Criodrilus, and 
perhaps of Geoscolex. 
3. The “ claspers” of Kynotus and of Siphonogaster, 
and perhaps of the Eudrilide. 
4, The tubercula pubertatis of the Lumbricide, Sparga- 
nophilus, Rhinodrilus, &c., which secrete a fluid and help 
to “ stick ”’ the two worms together.! 
In the case of the first three, specialised chet, copulatory 
1 The external muscular organ of Stuhlmannia variabilis is very ex- 
ceptional, and it is not quite clear in which group we should place it; 
possibly in the first, 
