Sm.i.As (hi the Origin f Fri'.thwntt r Faunas. 



97 



comparative rarity of Cordylophora mid its habit of growing attached to floating 

 timber, we nerd not regard its propagation by free-swimming larva as fatal 

 t-> our deduction. 



Tin- niarinc Polyzoa propagate exclusively by free-swimming larva; ; but in 

 the freshwater division of the group, statoblasts, similar in essential characters 

 ti 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 ex-apes, 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. 



Tnionacea, 

 < 'yprinacea, 



ISOMYA. 



f.O. 



f. m. 



Veneracea, /. m. 



1 1 HTEROMYA. 



/. m. 



Mvtilacea, 

 Mulleracea, 



Aviculacea, 

 ea, 



f.o. 



MONOMYA. 



LAMMELLIHRANCHIATA. 



Cyprinacea. 



Cardium, /. m. 



Cyrena, /. o. 



Cyclas, /. o. 



Pisidium, / o. 



Veneracea, 



Tellina (Galatea, /. o.) 



Scrobicularia. 



Mactra (Gnathodon, /. o.) 



Donax (Fischeria, /. o.) 



o. m. 

 o. m. 



Mytilacea. 

 Dreissena, 



f.o. 



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. \Ve cannot, therefore, discuss this group except in a very limited 

 manner. Commencing with the Lammellibranchiata, and of these first the 

 I'nionacea, 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 Pec-tens by the 



pa 



