Sottas—On the Origin of Freshwater Faunas. 97 
comparative rarity of Cordylophora and its habit of growing attached to floating 
timber, we need not regard its propagation by free-swimming larva as fatal 
to our deduction. 
The marine Polyzoa propagate exclusively by free-swimming larve ; but in 
the freshwater division of the group, statoblasts, similar in essential characters 
to those of the sponges, are usually developed. A well-known exception is that 
of Paludicella, in which statoblasts have not been observed. The development 
of the ovum in this genus, however, has been watched by Professor Allman, who 
shows that it takes place within the perivisceral cavity of the parent, so that 
when it escapes, by rupture as he conjectures, of the parental body wall, it 
is In a condition to settle down at once as a fixed or attached form. This excep- 
tion, therefore, may be said to prove the rule. 
LAMMELLIBRANCHIATA. 
Isomya. Cyprinacea. 
Integropalliata. Cardium, S.m. 
Unionacea, Pao: Cyrena, f. 9. 
Cyprinacea, fim. Cyclas, Aas 
Sinupalliata. Pisidium, So. 
Veneracea, Sut: 
Veneracea. 
Hereromya. _ Tellina (Galatea, /. 0.) 
Mytilacea, fim. Scrobicularia. 
Mulleracea, eid: Mactra (Gnathodon, /. 0.) 
Donax (Fischeria, /. 0.) 
Monomys. 
Aviculacea, 0. mM. Mytilacea. 
Ostreacea, 0. Mm. Dreissena, anos 
To give an account of the fresh-water Mollusca of the entire world would 
require a large monograph to itself; and this, when completed, would leave us in 
ignorance of many of those points relating to development which are essential to 
our inquiry. We cannot, therefore, discuss this group except in a very limited 
manner. Commencing with the Lammellibranchiata, and of these first the 
Unionacea, we meet in both genera of this family (Unio and Anodon) a most 
remarkable and well-known course of development, which furnishes a striking 
illustration of the necessity which exists in the case of slow-moving, freshwater 
forms of some means of dispersion not involving the production of a free-swimming 
embryo. The ova in these genera develop up to a certain stage within the gill- 
pouches of the parent, and remain there as ‘“ glochidia” till some passing fish 
or, it may be, a wading bird comes into their vicinity. The young glochidia 
then issue into the surrounding water, and, swimming like Pectens by the 
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