The Sexual Phases of 3Iyzostoma. 
259 
fuller account of these same species and adds several others. In 
all some ten gall-producing species were deseribed. The sex is 
mentioned in seven of these. Two of them [M. pentacrini and de- 
forrnator] are hermaphrodites and appear to agree with the free- 
living species except in the unilateral development of the testes. 
In one species [M. cysticolum) v. Geaff found two individuals, one 
large and the other veiy small, inhabiting a single cysi In the 
small specimen only male organs were seen; in the large one he 
found female' organs with vestiges of testes. In four other cysticolous 
species [M. tenuispinum ^ imllemoesii^ inßatoi' ^ miirrayi) there were 
also two individuals in each cyst, a large and a small one. Only 
male organs were found in the former and only female organs in 
the latter. v. Geaff therefore regarded these as dioecious species, 
but his mind seems hardly to have been definitely made up on this 
subject, as shown by the following remark ('84b, pag. 11— 12): »More 
abundant materials are required before the question about the life- 
history of the Myzostomida cysticola can be definitely answered, but 
my investigations permit me to state that the following is in all 
probability correct. 
The male and female being found associated in a common cyst 
and increasing in size with the growth of the cyst, shows that they 
perforate the arm-joints or pinnules of their host together. The growth 
of the cyst is of course caused by the presence of the parasite; the 
female deposits her eggs within the cyst, and the young embryos, 
after they have abandoned the cyst and lost their ciliated coat, 
associate together in pairs, and bore their way through the arm- 
joints. In both the sexual development begins with the appearance 
of testes (cf. M. hrevicirrum)^ but in the female the testes degenerate 
and disappear entirely, or leave but a minute rudiment [M, cysti- 
colum) when the ovaries make their appearance in additionb« 
^ V. Graff’s supposition that the young Myzostomes associate in pairs and 
together take part in forming a gall seems to me hardly plausible. Judging 
from my observations on M. glahrum and pulvinar^ both of which show a distinct 
tendency to occur in pairs, each consisting of a senior and junior individual, 
I believe that, in the case of the cysticolous species, the gall must be formed 
by a single individual, and that later a young Myzostome when it abandons its 
pelagic trochophore stage must enter through the aperture of the gall and settle 
down to a quiet life with the senior individual. The latter probably dies at 
the end ot its female stage and, undergoing decomposition, may perhaps serve 
as food for its still vigorous junior partner. This one in turn may thereupon 
become the senior partner of another young Myzostome, and so on. According 
Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. 12. 18 
