170 REVIEWS—CONTRIBUTIONS TO NATURAL HISTORY. 
from what are called their ovarian vesicles, what seem to be true 
medusze whether free or fixed. This remarkable observation caused 
some embarassment to speculative naturalists in determining the true 
gradation of animal forms. Since if Polypiferx be the lower class it 
is credible enough that the early stages of development of Acalephe, 
the next class in order, should bear a resemblance to Polypes, and 
vice-versa, but that two neighbouring classes should each in the 
young state assume the form of the other is antecedently improbable 
and is difficult to reconcile with our ideas of the order of nature. 
Our author proposes a novel view which completely relieves us from 
this difficulty, but which yet is not recommended to us for that 
purpose, but is supported by facts and by arguments of great weight, 
claiming careful consideration, and which we must say come near to 
convincing our minds. He maintains that Hydroid Polypes are 
imperfectly developed, or at least merely nutrient forms of the lower 
Acalephe. He shows that the anomalous forms of Acalephe are 
truly, in accordance with some of the most probable recent specula- 
tions, compound animals; that the floating colony contains Polype 
or nutrient forms along with medusan, which are reproductive 
forms, and he considers the Hydroid polypes as sedentary colo- 
nies of analogous character, either producing free meduse, which 
are the perfect and properly reproductive form of the animal, or 
having as a part of the colony fixed meduse forms, which fulfil 
the same function. Hence he transfers the Hydroid polypes alto- 
gether to the class Acalephe, where with the other inferior and 
often compound animals of that class they make the lowest order 
Hydroide. Two other orders are recognized : the Discophore, or 
typical meduse, and the Ctenophore, which are to be regarded as 
highest in the class. : 
It is a recognised principle that the presence of the polype form 
does not necessarily imply connection with the class Polypifera, as 
besides embryonic conditions of Acalephe we have the Polyzoa of 
much higher organization, and the Vorticellide belonging to a lower 
type, both simulating the Polypes. Mr. Dana had already marked the 
great importance of the structural difference between the Hydroid 
and Actinoid Polypes. It remained for Agassiz, by determining the 
real characteristics of Polypifera, and the true interpretation of the 
medusa forms produced by the Hydroids, to settle, we may at least 
say with the highest probability, the proper limits of the classes, 
giving the Hydroids their place among Acalephe, 
