LIFE OF JAMES DWIGHT DANA. 



work on alternate generations, and will find there that in 

 the extraordinary succession of alternate generations, 

 ending with the production of cercaria and its metamor- 

 phosis into distoma, a link was wanting the knowledge 

 of the young hatched from the egg of distoma. The 

 deficiency I can now fill. It is another infusorium, a 

 genuine opalina. With such facts before us, there is no 

 longer any doubt left respecting the character of all those 

 polygastrica ; they are the earliest larval condition of 

 worms. And since I have ascertained that the varticellae 

 are true bryozoa, there is not a single type of these 

 microscopic beings left which can hereafter be considered 

 as forming a class by itself in the animal kingdom. Under 

 whatever name and whatever circumscription it has ap- 

 peared or may be retained to this day, the class of Infusoria 

 is now entirely dissolved, and of Ehrenberg's painful in- 

 vestigations the descriptive details alone can be available 

 in future, but the whole systematic arrangement is gone. 

 This result has another interesting bearing; it shows the 

 correctness of Blanchard's view respecting planariae and 

 tneir close relation to the intestinal worm known under 

 the name of trematoda. Indeed, they belong to one 

 and the same natural group." 



AGASSIZ TO DANA 

 Classification of Crustacea 



" CHARLESTON, Feb. 9, 1852. 



" Many thanks for your very instructive remarks on 

 the classification of Crustacea ; they are the more welcome 

 since I pay as much attention as I can to that class now, 

 especially with the view of tracing their metamorphoses 

 in reference to classification. I have no doubt that the 

 principle which has guided you is identical or nearly so 

 in its results with that of embryonic changes. I would 

 offer a single suggestion. I do not know sufficiently the 

 specialities of carcinology to say positively that the 

 Cumce, as a group, must be suppressed, but I can state 

 with confidence that all the species of that genus which 

 I have had an opportunity to examine alive, and I have 



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