ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 315 
phroditc ; the ova are much larger than those of 0. virginica , and the 
young undergo development in the gill- and mantle-cavities ; in some 
points of structure they also present a resemblanco to the European 
species. 
Molluscoida. 
a. Tunicata. 
Blastogenesis in Botryllidae.* — M. A. Pizon has, in an elaborate 
memoir, chiefly directed his remarks to the development of buds and 
larvae, to asexual and to sexual reproduction. He has discovered in the 
Botryllidae two epicardiac tubes, formed by two posterior diverticula of 
the primitive vesicle, and analogous to those found in allied stalked 
Ascidians ; these tubes, however, are not isolated from the part of the 
primitive vesicle which gives rise to them, and, in the adult, they appear 
as the direct prolongation of the peribranchial sacs. The author con- 
siders that there has been a transportation of the blastogenetic faculty of 
the epicardiac tube of Polyclinidse to another portion of the endo- 
dermic vesicle of the Botryllidae ; in other words, to the external mem- 
brane of each peribranchial cavity. This peribranchial blastogenesis 
and the consequent disposition of the ascidiozooids in radiating rows is a 
natural consequence of the mode of attachment. The peribranchial 
cavity appears to arise in two different ways ; in Appendicularia , which 
is looked upon as the typical larva of Tunicates, the successive changes 
of the ancestral form have been such that the larvae do not undergo the 
least metamorphosis ; with the rest there has been retrogression, and 
so it happens that in some larvae the peribranchial cavities are formed by 
invagination of the ectoderm, while in others they are, as in one of the 
ancestral forms, diverticula of the primitive endodermic vesicle. 
M. Pizon has found that the vibratile organ commences as a dorsal 
diverticulum of the primitive endodermic vesicle, which appears at the 
same time as the peribranchial diverticula, the intestine, the heart, and 
the epicardiac tubes. This diverticulum then opens secondarily on the 
anterior part of the branchial vesicle, while it loses its hinder com- 
munication with the primitive vesicle ; on the side of this latter it under- 
goes a more or less rapid atrophy. It would seem then that the vibratile 
organ is not formed by a simple cul-de-sac of the anterior part of the 
branchial sac, as Van Beneden and others have thought ; consequently 
the homology with the hypophysis cerebri of Vertebrates, which they 
have suggested, cannot be maintained. The author believes that the 
vibratile organ represents the remains of an ancestral organ which 
played an important part in primitive Tunicates, but which has lost its 
functions in consequence of ontogenetic changes. He is of opinion 
that his views are confirmed by the homology which he has been able to 
establish between the different diverticula of the primitive vesicle of 
Crinoids and those of the primitive vesicle of the buds of compound 
Ascidians. He considers that the two peritoneal vesicles, the aquiferous 
vesicle and the endodermic prolongation of the stalk of Crinoids are 
respectively homologous with the peribranchial sacs, vibratile organ and 
epicardiac tube of Synascidians ; and if the doctrines of evolutionists be 
correct, we must believe in the existence of an ancestral form common to 
* Ann. Sci. Nat., xiv. (1893) pp. 1-386 (9 pis.). 
