1 882.] Zoology, ^(3 



{^From the American Naturalist, October, 1882.) 

 ZOOLOGY. 



Habits of Fresh- water Crustacea. — No one branch of bio- 

 logical study is now bringing forth more interesting and every- 

 way useful results than embryology. Throwing light as it does, 

 not only on questions of classification and theoretical biology, 

 but also on the application of such theories to practical life, 

 this new science may be termed at once the root and most 

 typical fruit of a revolutionized biology. No other science fur- 

 nishes a better illustration of the value of minute, accurate study 

 of the most common and apparently insignificant facts. Sets of 

 isolated facts evolved by conscientious study of different men 

 spring suddenly into line when once the clue is found, and the 

 result may be a new law which renders all these facts eloquent. 



To the systematist the merely external study of life histories is 

 of greatest value as a check against redundancy in classification, 

 and furnishes the only reliable method, among lower forms at 

 least, of setting the bounds of species. 



Many eminent monographers have been obliged to considera- 

 bly augment the nomenclature of their specialty with names 

 which, later, have proved to apply simply to larval or immature 

 forms, on account of the impossibility of following the whole life 

 history of each individual. 



To confine ourselves to the class Crustacea, many instances of 

 this sort could be recounted. The best known is perhaps that of 

 the common Cyclops which in the earlier days of carcinology 

 enjoyed as many as three names between its exclusion from the 

 e:gg and maturity. The discovery of the earlier stages in the life 

 of Cyclops opened a new vista in the whole subject, and now we 

 recognize a ** Nauplius stage " in the life-history of nearly every 

 crustacean. 



It has been more recently discovered that similar opportunities 

 for error are afforded by the difficulty of distinguishing the ulti- 

 mate stage in an animal's life. It has been shown that the func- 

 tions of reproduction are anomalous in the lower animals. Espe- 



VaSfm 



