4938 TuE ZooLocist—J UNE, 1876. 
hatched, within the cavities formed by the external gills; opinions 
differing, however, as to the manner in which these ova take up 
this position. The majority of authorities favour the opinion that 
they pass directly from the oviducts to the gill-cavities, and there 
remain, though it has been suggested by Von Hessling that the 
ova are possibly conveyed by means of the respiratory currents 
from one individual to another. Some observations personally 
made in reference to the reproductive phenomena of a marine 
representative of the group, the Modiola modiolus, would seem 
to indicate that Von Hessling’s hypothesis is by no means im- 
probable. 
Fine clusters of this bivalve have been long since established in 
the tanks of the Manchester Aquarium, whither they were imported 
with Alcyonium, Dianthus, and other zoophytes from the North 
Sea. On passing a tank containing a number of these one day last 
August, it was observed that dense volumes of granular matter 
resembling smoke were being ejected from the excurrent apertures 
of many individuals ; in some instances it was further noticed that 
the granular cloud was of a lighter colour and less dense con- 
sistency. ‘The depth of the tank being inconsiderable, a few drops 
of water containing samples of the two clouds were removed by 
means of a pipette, and, on examination with the aid of the micro- 
scope, yielded the results anticipated—namely, that the darker 
and more coarsely granular discharge was composed entirely of 
ova, while the lighter one consisted altogether of spermatozoa- 
By employing a still higher magnifying power— 4, in. Gundlach—it 
was likewise noticed that, where the two elements mingled with 
one another, the spermatozoa readily attached themselves to the 
ova, a single ovum in many instances bearing five or six or more 
of these rod-like bodies. This attachment was altogether irregular, 
no distinct micropyle being detected. 
Pressure of other matters at the time interfered with the pur- 
suance of the subject beyond this identification of the sexual 
elements, and the manner in which they were brought together to 
insure the fecundity of the ova. If the succeeding phenomena of 
development of Modiola agree with those which obtain in Anodon 
—that is, within the gill-chambers of the parents—it would be 
requisite for the ova, after fertilisation in the outside waters, to be 
again drawn within the parent’s shell by the inflowing respiratory 
currents. This explanation is quite in keeping with the hypothesis 
