1890.] Annelid Descent. 1149 
of the Annelid, retained the ability to produce egg or sperm 
mother cells; while the remainder, at first a sort of indifferent 
follicular epithelium, was pressed against the internal organs and 
tissues, and finally surrounded them in the form of a peritoneum. 
In this process the hzemal and neural mesenteries and the septa 
came into existence, as the previously existing dorso-ventral 
parenchym muscles became enclosed between the median walls 
of a pair of segmental cavities, and between the anterior and pos- 
terior walls of two successive ones, 
The greater part of the primary body-cavity, consisting in the 
ancestral forms presumably of a lymph system of irregular holes 
and clefts in the parenchyma, was filled up for the most part by 
the expansion of the sexual follicles. Only a small part of it re- 
mained as the definitive vascular system. Since the coelom sacs 
at first were rounded, they would not apply their walls immedi- 
ately to the entire surface of the intestine, skin and to one 
another, but would leave open certain definite spaces —namely, 
intersegmental circular spaces, lying transversely beneath the in- 
tegument; a median space above and below the intestine, com- 
municating each with the circular space, and lying between the 
right and left lamella of the mesentery. Joined to these there 
was also the above-mentioned intestinal sinus. Thus the method 
of origin of the segmented secondary body-cavity depicted 
above would at the same time have led to the formation of the 
chief portions of the vascular system, as a naturally resulting con- 
seqgience of the given spacial relations. 
Among the peritoneal structures of the Annelids, the neural 
and hemal longitudinal muscles require special attention from 
the difficulty of divining the causes of their first appearance. I 
have formed the following as yet very hypothetical conceptions of 
these causes. Part of the non-reproductive elements of the wall 
of the sexual gland I regard as epithelio-muscle cells, the bodies 
of which were in the epithelial layer of the follicle wall, while 
the distal parts elongated as fibrils at each end and tangential 
to the surface of the gland, when contracting exercised pressure 
upon the contents of the follicle cavity (were thus functional 
originally in the discharge of the sexual products). After the 
