96 SOLLAS On the Origin of Freshwater Faunas. 



of those sponges* which are found inhabiting fresh water. In marine sponges, on 

 the other hand, multiplication by free-swimming larvee is the rule ; and when, as 

 in some cases, gemmules are produced as well as larvae or in place of them, 

 they are generally external ; and if internal, never, so far as is known, remain 

 within the parental tissues for any considerable time, but are set free as soon as 

 produced as free-swimming organisms. Moreover, they are never provided with 

 the characteristic protective envelopes which characterise the statoblast of the 

 freshwater sponge. 



Thus it appears that the evidence of the Spongillina may be claimed as sub- 

 stantiating, in a very high degree, the truth of our views. 



We turn next to the Hydrozoa ; and, as the development of the free-swimming 

 Limnocodium is unknown, we must restrict ourselves to the Hydroids, of which, 

 as far as at present known, there are only two freshwater genera, Hydra and 

 Cordylophora, both belonging to the same family, the Tubularidse. In Hydra the 

 egg is surrounded by a horny case, within which it undergoes its development. 

 There is thus no free-swimming stage, and the young Hydra hatches out in the 

 adult form. While Hydra, which is by far the commonest freshwater Hydroid, 

 thus favours our view, the other genus, Cordylophora, presents us with what 

 appears at first sight a most damaging exception, for the young of this Hydroid 

 are liberated in great numbers as free-swimming planulse, which persist in a state 

 of great locomotive activity for some time before settling down to a sedentary 

 existence. This exception is otherwise remarkable, as affording, according to 

 Semper, the only known instance in which the progress of the colonization of 

 freshwater streams by a semi-marine genus can be historically traced. The 

 discovery of Cordylophora was made by Allrnan, who found it in the Grand Canal 

 Docks of Dublin in 1854. Since then it has migrated into many rivers, and has 

 already reached the Seine at Paris. One would like to be sure, however, as to 

 how much of this history represents the progress of the discovery of the genus and 

 how much its actual migrations. It is especially interesting as throwing light on 

 the means by which Cordylophora has been enabled to carry on its migration, to 

 find it frequently associated with Dreissena polymorpha, a mollusc which has, in 

 recent times, colonised many rivers in England. Both Dreissena and Cordylophora 

 are attached animals, frequently found growing together on floating timber ; and 

 it is probably owing to the occasional transport of this up stream by human 

 agency that the introduction of both these forms into our rivers is due. It is also 

 possible that the larvae of Cordylophora, which is known to grow attached to 

 buoys at the mouth of the Elbe, may attach themselves to boats at their moorings, 

 and subsequently be transported by them up stream. Taking into account the 



* For much valuable information and suggestive remarks on this subject I would refer to Papers by 

 Carter and Marshall, in the Annuls and Mayazine of Xatural History, ser. v., vols. xii. and xiii. 



