720 SUMMARY OF CUERENT RESEARCHES RELATING TO 



out to sea. Hence, frest-water animals should not, as a rule, pass 

 through a free larval stage of existence, nor, as a matter of fact, do 

 they. In Hydra, fresh- water sponges, and Polyzoa, the young usually 

 emerge from a horny cyst in the complete state. In the Unionidse, 

 the glochidium-stage provides for distribution without involving a 

 seaward journey. The young of fresh-water molluscs do not enter 

 upon a free existence till they are similar to their parents, and 

 Paludina is viviparous. The suppression of a free-swimming larval 

 stage not only occurs in fresh-water, but in many marine invertebrates. 

 This is connected with the fact that the larval stage is in a position 

 of disadvantage as compared with the adult. Hence there is an 

 advantage to the organism if the larval stage can be passed over in a 

 state of seclusion. From this various other modifications follow ; 

 development in seclusion involves a supply of accessible food, hence 

 the appearance of yolk and other kinds of nourishment furnished by 

 the parent to the imprisoned embryo. Again, the secluded larva 

 being spared the drudgery of working for its own existence, and 

 supplied with nutriment in a form that puts the least tax on its 

 digestive powers, a larger balance of energy remains available for 

 metamorphic changes. Thus arise the phenomena of accelerated and 

 abbreviated development. Further, the shortening of the larval life 

 probably leads to the lengthening of the adult life, and shifts the 

 chances of variation and selection forward into the adult stage. Thus, 

 animals which hatch out in a complete state will most probably suffer 

 modifications of that, and not of previous ones, except very indirectly. 

 Here we discover a direct tendency towards a mode of development 

 which explains the " arborescent " character of our zoological classifi- 

 cations, i.e. the tendency of the tree of life is now to produce leaves 

 rather than new branches. In the case of fresh- water faunae very 

 direct reasons have existed for the suppression of the free larval 

 stage. In this connection may be noticed the richness in species and 

 the poverty in genera of the fresh-water moUusca. 



In discussing the origin of fresh-water faunae there are three 

 hypotheses from which we have to select : (1) that marine forms have 

 migrated into rivers ; (2) that they have migrated into marshes, and 

 thence into rivers; and (3) that marine areas have been converted 

 into fresh-water ones. The last course has been the most usual, 

 especially in the case of non-locomotive forms. Hence the origin of 

 fresh-water invertebrates is connected with the great movements 

 which have affected the earth's crust. 



Pelagic Fauna of Fresh-water Lakes.* — 0. E. Imhoff first deals 

 with the " Langensee," and refers to the remarks of Crisp as to the 

 synonymy of some of his species with those previously described by 

 Gosse, pointing out certain errors or lacunae in Gosse's descriptions. 

 Dealing with the pelagic fauna of four of the lakes of northern Italy 

 he adds to them one Flagellate, Dinohryon divergens Imhoff, a species 

 of Ceratium, Gonochilus volvox, Anurcea cocMearis and longispina, 

 Asplanchna helvetica, and a species of Polyarthra. Among the 



* Zool, Anzeig., vii. (1884) pp. 321-7. 



