288 
W. M. Wheeler 
geniis ('88, pag. 61). Both Myzostoma Spinther have arisen from 
Polychaete ancestors, but both have taken to parasitism — Spinther 
to living on Sponges and Myzostoma on Crinoids — with the result 
of an interesting parallelism in structure. Although Spinther has 
retained a greater number of Segments (from 12 in S. mmiaceus to 
52 in S. arcticus) than Myzostoma^ both forms have tended to depart 
from the elongate Annelid type and to assume a flattened form and 
more radial symmetry. In Spinther the reproductive orifice lies in 
the mid-dorsal line just above the anus and the intestine sends out 
a pair of diverticula in each segment except in the posterior end of 
the body. »Endotheliale Septa zwischen den einzelnen Segmenten 
sind nicht vorhanden und nur die zwischen den Darmdivertikeln 
verlaufenden dorsoventralen Muskelbündel stellen eine unvollständige 
Kammerung des Leibesraumes dar.« The parapodia with their hooked 
setae and dorsal cirri, and the general appearance of the body-cavity 
with the sexual products accumulated in the portion dorsal to the 
intestine recall the conditions seen in Myzostoma. Spinther has no 
nephridia according to v. Graff, and he has not succeeded in find- 
ing the ovaries and testes, although it is possible that certain clusters 
of large »indifferent cells« in the ventral portion of the body-cavity 
between the intestinal diverticula and between these and the Integu- 
ment may be the Organs in question. 
Resume and Conclusion. 
For the sake of a more graphic expression of the sexual phases 
of Myzostoma I give two diagrams, PI. 12 Figs. 57 and 58; the former 
representing the conditions in species like M. cirrijerum and glahrum^ 
the latter in M.pulvinar and probably many of the Cysticola which have 
been supposed to be dioecious. To begin with Fig. 57, the whole 
triangle ahm, may be taken to represent the whole of the reproductive 
System from its first appearance in the embryo, or trochophore (at d) 
to its development in the adult, when it may be represented by the 
line hm. The unshaded triangle ahc may be regarded as the sexless, 
neutral, or undifferentiated condition of the reproductive cells (gono- 
blasts) and the two parts of the line viz. ho and oc as the ulti- 
mate division into spermatogonia on the one hand and oogonia on 
the other. The dark triangle hoh represents the male, the paler 
trapezium cohm the female organs. The lighter region hoed of the 
male triangle represents the period preceding the appearance of the 
