The Sexual Phases of Myzostoma. 
231 
called ))penes((. These latter organs do not lie in the piane of the 
seetion fìgured. 
The sections thus far described for the purpose of orienting the 
reader show , only that specimens of M. cirriferuìn are hermaphrodite. 
i. e. contain both mature ova and spermatozoa side by side. These 
sections represent the condition which obtains during the greater 
part of the animai' s life. For further details and clearer fìgures 
the reader may be referred to v. Graff's beautiful monograph ('77), 
in which the hermaphrodite phase of M. cirriferum is correctly 
described. 
The facts on which I wish to lay particular stress are only brought 
out by the study of an unbrokeu series of individuals of J\L cirri- 
ferum of different ages, and I venture to maintain that had such a 
study of this one species been undertaken earlier, much of the con- 
fusion which, has arisen respectiug the sexual phases of Myzostoma 
would bave been avoided. The account bere given is based on a 
large number of specimens, but for the sake of brevity I bave con- 
structed what I consider to be a typical series from somewhat 
fluctuating measurements. Such variations are uuavoidable because 
the animals suffer a varying amount of contraction during killing 
and preservation, and because the changes in the reproductive organs 
often make their appearance somewhat earlier in some individuals 
.than in others. 
Stage 1. 0.225 mm long. In Myzostomes of this stage the 
intestine is very simple in its structure, consisting of a straight 
median tube with three very short diverticula on either side. There 
is but a single pair of ovaries consisting of oogonia. The testes 
are present as larger masses of spermatogonia, ventral to the short 
intestinal ramitìcations. A few of the spermatogonia are dividing, but 
there are as yet no spermatozoa. 
Stage 2. 0.25 mm long. The intestine has undergone little 
change. There is no= body-cavity. The ovaries are more distinct 
and consist of a triangulär mass of cells on either side wedged in 
between the base of the intestine and its incipient branches. It is 
possible to distinguish two kinds of cells intermingled in the ovary, 
the one with larger clearer nuclei sometimes in karyokinesis, the 
other with smaller more deeply staining nuclei. The former are the 
oogonia and youngest oocytes, the latter I shall designate as ac- 
•cessory cells. The dividing spermatogonia in the testes are more 
numerous than in stage 1, but stili there are no mature spermatozoa. 
