AND PHYSIOLOGY OP THE SPONGIAD^. 
817 
monad-like bodies around them. As they enlarge in size they become oval-shaped, and 
at length in their mature state they acquire a regular ovate form.” When they have 
attained a fully-developed condition, they separate from their attachment to the parent 
and pass out of the fsecal orifices. At this period of their existence the learned author 
states that they are endowed with spontaneous motion, in consequence of their larger 
extremity being furnished abundantly with cilia, which the author describes as “ very 
minute transparent filaments, broadest at their base, and tapering to invisible points at 
their free extremities.” After floating freely about for a period, they attach themselves 
to some fixed body, adhering firmly to it, and spreading themselves out into “ a thin 
transparent convex circular film.” The author further states that “when two ova in the 
course of their spreading on the surface of a watch-glass come into contact with each 
other, their clear homogeneous margins unite without the least interruption, they thicken, 
and produce spicula : in a few days we can detect no line of distinction between them, 
and they continue to grow as one ovum.” 
I have never had the good fortune to see the living gemmule with its cilia in action, 
as described by Dr. Geant, but I have frequently found Halichondraceous sponges 
with an abundance of these gemmules attached to their tissues ; and I have in my pos- 
session a beautiful little specimen, dredged off Shetland, for which I am indebted to my 
kind friend Mr. Baelee, which is very illustrative of Dr. Geant’s description of the mode 
of the development of the young sponge after the ovum or gemmule has attached itself. 
On a fragment of a bivalve shell there are more than twenty or thirty of Dr. Geant’s 
ova or gemmules, which are all in the same early stage of development, each forming a 
small group of extremely slender spicula. The groups are separate from each other, but 
very closely adjoining. The diameter of one of the largest does not exceed -g-^th of an 
inch, and theh distance from each other is about half or once the diameter of one of them. 
In their present state, as represented by six of them in Plate XXXIV. fig. 16, it is 
evident that they are separate developments, and it is equally evident that a slightly 
further amount of extension would have caused them to merge in one comparatively 
large flat surface of sponge. We see by this instance that a sponge is not always 
developed from a single ovum or gemmule, but, on the contrary, that many ova or 
gemmules are often concerned in the production of one large individual, and this fact 
may probably account for the comparatively very few small sponges that are to be found ; 
a few days probably serving by this mode of simultaneous development to form the basal 
membrane of the sponge of considerable magnitude, as compared with the individual 
ovum or gemmule, or with a sponge developed from a single ovum only. This mode 
of reproduction appears to have a very wide range. It is common to several distinct 
genera of Halichondraceous sponges ; and I have observed it also in a siliceo-fibrous 
sponge, Ij^hiteon jpanicea of the Museum of the Jardin des Plantes, Paris. Fig. 17, 
Plate XXXIV. represents a small piece from the interior of the skeleton of Iphiteon 
jpanicea. Although the latter sponge is so widely different in structure from the Hali- 
chondraceous tribes of sponges, its mode of propagation by gemmation seems to be in 
