112 Sotias— On the Origin of Freshwater Faunas. 
is not opposed to this view, as the water in such tanks is maintained at the tempera- 
ture of the laboratory, and not at that proper to the season. 
On the origin of the glochidium in the Unionide it is, in the absence of data, 
almost useless to speculate. One might suggest that it is the specialised descendant 
of a form of larvz once of very general occurrence, and of which, as the develop- 
ment of the Mollusca becomes better known, some traces may still be found to 
exist. Its marked persistence in the Unionide would be clearly owing to modifi- 
cations giving it a functional importance as a means of distribution. 
The causes which have led to the viviparous character of Paludina and the 
attachment of the eggs of other freshwater Mollusca and Crustacea need special 
discussion, and we must now enter upon a somewhat lengthy argument, though 
I fear it will not appear so extensive and detailed as its importance demands. 
I will begin by stating the following proposition:—The higher the position 
an animal occupies in the scale of organised beings, the higher, as a rule, the stage 
at which it assumes a free existence. Thus the lowest classes of animals, such as 
the sponges, ccelenterates, and echinoderms, almost invariably commence an 
independent existence as free-swimming larvee, very unlike the parent in outward 
form and general structure. So do most of the Vermes, including the whole of the 
Brachiopoda and Polyzoa. 
The Mollusca, Arthropoda, and Vertebrata, exhibit instructive gradations in 
this respect. 
The young of all classes of the Vertebrata are always born in full possession 
of the characters distinguishing that sub-kingdom. (I here follow Balfour in 
regarding Amphioxus as a member of the Chordata, but not of the Vertebrata.) In 
the Icthyopsida, the lowest division of the Vertebrata, the young, however, frequently 
undergo a remarkable metamorphosis before assuming the adult state; at the same 
time they always enter upon existence as evident Icthyopsida. In the higher sub- 
divisons Sauropsida and Mammalia, no considerable metamorphosis takes place, and 
the young are always born with the same generic or at least family characteristics 
as distinguish their parents. In the Mollusca we find the lower forms, such as the 
Chitons, commence life as free-swimming larve : so do some of the Gastropoda, but 
others do not. Thus the first hatched young of Murex, one of the culminating 
genera of the marine Gastropoda, are in all important respects similar to the parents. 
The case is similar with the Lammellibranchiata, the young being sometimes born 
as free larvee; at others as young adults. But in the Cephalopoda, the crown of 
the molluscan sub-kingdom, the young animals never are hatched otherwise than 
as young cuttlefish. In the Arthropoda a similar gradation exists. The lower 
orders of Crustacea are characterised by passing through a free nauplius stage ; 
the higher, such as the Decapoda (except in the case of Peneus), are not hatched 
until they have reached the zoéa stage, or it may be until they have completed 
their development, as in the case of the lobster. 
