﻿April, 1857.] 



ELLIOTT SOCIETY. 



119 



Mesonema seems to indicate another complication of the embryo- 

 logical plan by the introduction of fissiparition. 



This direct metamorphosis among Aeginidae, is of lower rank 

 than the alternate generation or individualized metamorphosis of 

 the Campanularians and Tubularians. If we bear in mind the 

 condition of the larva in its earlier stages among Campanularians, 

 when producing only planules, we find that it is a free swimming 

 embryo until it is ready to assume its polyp form, when it loses 

 its power of locomotion and becomes fixed for life. This ciligrade 

 movement, then, is the index of a low condition, and among the 

 Aeginidae we find, that the existence of this ciligrade locomotive 

 condition is prolonged in Aeginopsis, until the Medusa form is 

 assumed and in Stenogaster and Cunina, though probably it exists 

 for a shorter time, yet the young hydroid never becomes fixed but 

 only assumes that condition which we find in the larva of Tu- 

 bularia, when with developed tentacula and freed from the bell of 

 its medusa, it moves about by means of the tips of these tentacula, 

 and selects a site to spend its remaining existence. Hence, when 

 compared with the remaining Hydroid medusae, the larval condi- 

 tions of the Aeginidae, are of low type, and we should not exclude 

 from this consideration the fact, that they are in two instances 

 parasitic in their character. If we compare this conclusion from 

 embryology with that which we should derive from their low 

 structure, we find that both would equally prove them to be the 

 lowest of the Medusae. At the same time the gentle gradations 

 which superior knowledge is leading us to conclude, exists be- 

 tween the Aeginidae on the one hand, and Eucope and Thauman- 

 tias on the other, gives weight to the belief that there is no essen- 

 tial difference of type between the direct metamorphosis of the one 

 and the individualized metamorphosis of the other. In both at a 

 particular stage, multiplication by gemmation takes place — but 

 in one the buds are all of the same kind, and all become medusae — 

 in the other the buds originally similar, are gradually differentiated 

 into different classes of individuals. And this latter mode is evi- 

 dently only a development by specialization of the one fundamen- 

 tal plan of the embryology of the Hydroid Medusae. 



It seems pretty clear from the observations of Prof. Agassiz, 

 (Boston Soc. Nat. Hist. Proc, vol. 3, p. 354,) that the fresh water 

 Hydra produces free Medusae. But having never had an opportu- 

 nity of repeating this observation, I cannot give more than a con- 

 jecture as to the relations of this Medusa to the others. Prof. Agassiz's 

 very short account, however, seems to indicate a medusa of nu- 



