ZOOLOGY. 



well as insects. Prof. Agassiz had himself, within a few weeks, had an oppor- 

 tunity of studying the embryology of a species of shark (Acantheus Ameri- 

 canus). He had found the yolk, not surrounded by an amnios, but resting in 

 the centre of an area vasculosa, and presenting, in its early development, 

 other peculiarities only known to exist in the egg-laying vertebrata. He 

 considered Plagiostomes a distinct class of animals from fishes, and he 

 thought it probable that Cycbstoiues should also be separated as a class. He 

 could not refer to one class animals developed in such different modes. 

 The number of classes into which the animal kingdom is divided into six 

 by Linnaeus, into sixteen by Cuvier, and into twenty-nine by Ehrenberg 

 shows that anatomical differences are insufficient for a proper determination 

 of classes. He proposed that the general plan of structure be a test for 

 types, and the manner in which this plan is developed the test for classes. 



Prof. Agassiz, in alluding to the probability of a fecundation of the egg 

 whilst in the ovary, stated that Dr. Weinland had found, in the viviparous 

 Zoarces anguillaris, that the ovarian bag (Graafian vesicle) of the mature 

 eggs was not a simple sac, but a double one; and, further, that this double 

 sac was not continuous over the complete circumference of the egg, but 

 that a disc of considerable size remained uncovered at the upper part, 

 where the spermatozoa might come in contact with the 3'olk membrane. 

 Dr. Weinland had also found the same condition in the skates and turtles. 

 Prof. Agassiz thought that the same organization would be found in all 

 vertebrata. 



FECUNDITY OF CERTAIN FRESH-WATER MOLLUSCS. 



At a meeting of the American Academy, May, 1859, Mr. Isaac Lea exhib- 

 ited some remarkable specimens of Unionidse, six to eight inches wide. 

 Some of these had the soft parts, and he called attention to two fine spe- 

 cimens of Margaritana complanata and Unio multiplicatiis, both females, 

 with the oviducts fully charged with embryonic shells, ready to be dis- 

 charged by the parents. There were two important points he wished to be 

 noticed. 1st. The enormous quantity of young in the mass of the outer 

 branchiae of the Margaritana (the branchiae were 3 X U X \ inches), each 

 specimen containing probably several millions of individuals. 2d. That the 

 Unio multiplicatiis was peculiar in having both lobes on both sides charged with 

 embryonic shells, containing no doubt several millions of individuals. This 

 species of the Unio is the only one Mr. Lea had ever observed furnished with 

 oviducts in all the four lobes of the branchiae. It is very probable that half 

 a dozen of these Mollusca would produce individuals equal in number to the 

 population of the whole United States. 



ON THE REPRODUCTION OF PARASITIC ANIMALS. 



At a meeting of the Boston Society of Natural History, Dr. D. F. Weinland 

 gave an account of the Reproduction of Parasitic Animals, and explained 

 the phenomena of alternation of genei-ation in the parasitic Trematoda of 

 the freshwater snails. The first generation of this animal exists, in the 

 form of " Distoma," in the intestines or lungs of vertebrata, as a perfect 

 animal with genital organs producing eggs; the next generation exists as a 

 yellow worm in the liver of the snails mentioned above, with or without an 

 intestine, their bodies being filled with the third generation of the animal, 



