128 AMERICAN JOURNAL 



aquatic, but live a long time in the air ; Anabas, Land-crabs, 

 Anthocole, Hirudines, Neritina, Littorina. 



Endoparasites (Entozoa), living in the juices of terrestrial 

 animals, or burrowing in their tissues, may be considered 

 aquatic animals. 



3. Aquatalia, animals whose existence entirely depends on 

 ■water ; Fishes, Crustacea, Worms, Branchiferus Mollusca. 



.Pelagic animals are generally gelatinous, pellucid, and need 

 in most cases no hard covering, as they rarely come in contact 

 with hard bodies; except species living on sea- weeds, which may 

 be considered islanders. They are either stoimming, as Cepha- 

 lopoda, Acalepha, Pteropoda, Heteropoda, or floating, as Jan- 

 thina, finless Heteropoda, Sagitta and Salpge. 



" In comparing animals of the same natural group, we always 

 find the terrestrial to be more perfectly organized than the 

 aquatic." — McLeay. Agassiz has treated this question most 

 elaborately in a paper entitled " The Natural Relations between 

 Animals and the Elements in which they live."* 



XIII. Zoogeographical provinces and geological periods afford 

 Bome of the safest guides in discovering natural aflBnity. Ter- 

 restrial and fiuviatile animals are often divided according to 

 their habitation in the old and new world, into very natural 

 groups, — e. g., Quadrumana, Scansores, Marsupialia, Edentata, 

 Humivaga3. Nearly all divisions of lind and fresh water Mol- 

 lusca agree with zoogeographical provinces. f The circumpolar 

 regions show no notable difference between species, being in- 

 habited by closely allied species of the same genera. 



Swainson has fii'st pointed out that the typical groups in each 

 division are European. | The reason cannot be that the European 

 species are more familiar to most authors, but, in reality, animals 

 of the new world have nearly always characters considered to 

 indicate inferiority ; for instance, the long tails of American 

 Monkeys and Parrots; greater number of teeth of Platyrhina. 

 ''Tropical forms stand generally highest in their respective 

 classes." — Agassiz, 1. c. p. 121. "Species of generally cold 

 climes are inferior." — Dana, Am. Journ. of Science, 1863, p. 

 330. No mammal with a thumb is found in the northern hemi- 

 sphere (Kaup). Alf. De Candolle considers the lower plants 

 more widely distributed than the higher ones, but this arises 



* American Journal of Science and Arts, ix. 1850, p. 369, and Annals 

 and Mag. of Nat. Hist. 



t Morch, iVlalacozoologische Blatter, 1859, p. 102. Journal de Con- 

 chyliologie, 1865, p. 265. 



X Europe is in zoogeographical respect limited by Sahara and Hima- 

 laya. Falaiarctic region of Giinther. 



