10 
THE VOYAGE OF H.M.S. CHALLENGED. 
Myzostomida except Myzostoma willemoesii). It would be desirable, however, to place 
this beyond a doubt by the help of sections, since it is always possible that (except in 
Stelechopus and the encysted species) there may be microscopic rudiments of suckers 
remaining ; and, on the other hand, it is possible that certain cavities on the ventral 
surface of many species do not really represent suckers at all, as they were formerly 
supposed to do. It seems also the limit of the suckers and their appearance generally 
varies according to the different state of contraction in which they are. 
The shape and arrangement of the suckers is also of importance for classificatory 
purposes. In Myzostoma calycotyle (PI. III. figs. 25, 26) there are stalked suckers, 
which have a very singular relation to the parapodia, being situated quite close to their 
external sides, commencing from the middle line of the body. The general rule is that 
they occupy the middle of the interval between two parapodia. 
Alimentary Canal. 
I have already spoken of the general configuration of the alimentary canal, and its 
influence on the outer form of the body. It remains to be stated that there are species 
{Stelechopus) in which the alimentary canal, instead of being ramified and divided into 
stomach, intestines, &c., is simple and straight, with only feeble indications of lateral 
branches. This peculiarity, accompanied as it is by other important variations from the 
typical structure, is of great use for systematic purposes. 
Generative Organs. 
The suggestion made by v. Willemoes Suhm that some Myzostomida were in all 
probability dioecious, has been amply verified by my investigations, and I have also to 
add to our knowledge of the group many facts concerning the structure and disposition 
of the organs themselves. The following is a general account of the structure of these 
organs, leaving out the genus Stelechopus, which is but imperfectly known. The cloacal 
aperture is situated on a papilla, and is the common opening for the rectum and oviduct. 
The male sexual openings are two, corresponding to the number of the testes ; they 
open on the ventral surface of the body, one on each side, between the third para- 
podium and the margin of the body. The apertures are sometimes simple, but some- 
times their borders are prolonged into a tube-like continuation which is very contractile, 
and may assume therefore very different shapes even in the same species. The male 
apertures are absent, or only present on one side in the Myzostomida Cysticola, 
which is owing to the fact that the testes in this group are either absent or unilaterally 
developed. 
