12 
THE VOYAGE OF H.M.S. CHALLENGES. 
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 appear- 
ance of testes (cf, Myzostoma brevicirrum, p. 43), but in the female the testes degenerate 
and disappear entirely, or leave but a minute rudiment ( Myzostoma cysticolum), when the 
ovaries make their appearance in addition. The curious shape of the female — convex 
below and flat or concave above — is owing to the growth of the cyst, whose sides rise 
round the parapodia and press the lateral parts upwards. 
Among the hermaphrodite forms, e.g., Myzostoma pentacrini and Myzostoma defor- 
mator, it is impossible to say anything for certain about the relations between the indivi- 
duals enclosed in one cyst. As three, two, and sometimes only a single individual are found 
in a cyst, it is evident that self-fertilisation must sometimes occur ; and the fact that when 
more than one individual is found in one cyst they are separated by partitions to a 
smaller (. Myzostoma deformator ) or larger ( Myzostoma pentacrini) extent, tends to show 
that it is the rule. In this case, therefore, it is by no means necessary that several 
individuals should be, associated together in a common cyst, and the occurrence must 
be regarded as accidental. 
Relation of Myzostomida to their Host. 
The Myzostomida Cysticola are interesting to the zoologist and paleontologist, not 
only from their structure but also from the cysts they produce, which recall plant galls. 
Under the description of species, a detailed account of these structures and the frequency 
of their occurrence will be given. I may here briefly allude to the most important facts. 
The effect of the free-living Myzostomida on their host I have already discussed in my 
former Monograph, but I will add that I lately received from Naples, through the 
kindness of Dr. Spengel, a specimen of Antedon rosacea, bearing numerous examples of 
Myzostoma cirriferum upon its arms, and not less than sixteen large specimens of Myzo- 
stoma glabrum upon the disk. 
Myzostoma asymmetricum is somewhat transitional between those forms that crawl 
freely about their host and those that are encysted. It is found attached to the ventral 
surface of the pinnules, which becomes slightly enlarged by the contact of the parasite, as 
does also the proximal arm-joint (PI. XI. figs. 4-6). Myzostoma willemoesii causes a more 
marked malformation (PI. XIV. figs. 6, 7) ; it is attached to the ambulacral surface of the 
pinnule, the joints of which become larger and more hollowed out, forming thus a canal ; 
the whole pinnule is wound spirally, forming a chamber in which the parasite lives, which is 
closely similar to that produced by Pemphigus bursarius upon the stems of poplar leaves. 
Myzostoma deformator bores its way into the interior of the pinnule, which becomes 
swollen and pear-shaped (PI. XII.). Another kind of malformation is produced by the 
two species Myzostoma pentacrini and Myzostoma tenuispinum. The former causes insigni- 
ficant thickenings of the arm-joints (PI. XI. fig. 9) and fissures between them. Much more 
