ON FRESHWATER POLYZOA. 823 
moniliform manner. As they increase in size the membrane of the ovary 
becomes strained over them, and they finally rupture it and escape into the 
perigastric space, where they lie loose in the surrounding fluid. 
Just before the chord-like organ becomes attached to the walls of the cell, 
it presents in many cases an enlargement which seems due to a peculiar in- 
vestment acquired at this spot, and which is quite distinct from the ovary: 
this structure I believe myself justified in viewing as a testicle. M. Van 
Beneden believes in the existence of a testicle occupying in certain polypides 
of Alcyonella fungosa, the position of the ovary in others, and is thus led 
to maintain the existence of distinct male and female individuals in the same 
colony*. I confess however that my own observations do not tend to con- 
firm the view of the distinguished professor of Louvain. The ova, on esca- 
ping from the ovary, are in most species, perhaps in all, still enclosed in a 
delicate transparent membranous investment, which however is soon lost. 
The general formation of all these ova, when they have arrived at maturity, 
is that which results from the apposition by the concave surfaces, of two 
concavo-convex horny discs united in all cases, except in redericella, by an 
annulus of a different structure which runs round the entire margin over- 
lapping each disc. The ova in the different species vary from a lenticular 
shape to an elongated oval, and in F’redericella the marginal ring is obsolete. 
In all, one surface would seem to be a little more convex than the other. 
In Cristatella the mature ovum is furnished with hooked spines, which 
spring alternately from the two sides just within the annulus, and thence 
passing outwards over the latter project in short rays beyond the margin. — 
The disc in all the species is of a deep brown colour, and would seem to be 
composed of a single layer of hexagonal cellules, whose external walls in 
most cases slightly project beyond the surface of the disc, and thus give to 
the latter an elegantly mammillated condition. In some cases however the 
cellular condition of one or both discs is very obscure. The annulus is also 
_ composed of cellules, which here, however, occur in several layers; these 
cellules are also for the most part larger than those of the disc and of a dif- 
ferent colour; they are filled with air, and give to the annulus a light spongy 
texture. ; 
In Cristatella I have succeeded in tracing certain stages in the progress of 
the ovum towards the mature condition, in which it is ready to escape from 
the body of the parent. In an early stage it may be observed as a whitish 
semitransparent compressible vesicle enclosing a fluid loaded with granules or 
minute cellules. In this state its surface is perfectly smooth, but we find it 
before long acquiring two additional investments, which however possess but 
a temporary existence and are destined to disappear as the ovum advances 
to maturity. The internal of these is a thick layer of a gelatinous con- 
sistence, which immediately invests the ovum; the external is a delicate 
transparent membrane which retains the internal gelatinous investment in 
its place, and is thickly covered on its outer surface with minute vibratile 
cilia. The action of these cilia seems to be confined to the production of 
currents in the surrounding fluid, and is thus probably subservient to the 
function of aération, for they are evidently tou weak to act as organs of 
_ locomotion, at least I never witnessed the ovum carried about by their aid 
through the surrounding water. ‘his interesting condition of the ovum 
must be carefully distinguished from a ciliate locomotive embryo, exam- 
ples of which will be presently adduced. Within the gelatinous envelope 
the ovum acquires its horny shell and annulus, and has now attained to its 
full size, still invested by the gelatinous and ciliated envelopes, but as yet no 
* Quelques Observations sur les Polypes d’eau douce, loc. cit. 
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