ZOOLOGY. 333 



transverse partition of the animal, a circular and radiating set of mus- 

 cular fibres can be made out, the latter assisting in moving the eye- 

 specks. Prof. Agassiz has also completed the study of the nervous 

 system in Medusae. He has found a continuous nervous thread extend- 

 ing from one eye-speck to another. These organs present a closer anal- 

 ogy to the organs of vision in the higher animals than has heen gen- 

 erally supposed. In their structure, some of them united in clusters 

 closely resemble a vertical section of the compound eyes of insects, 

 and probably are capable of receiving an impression of light and dark- 

 ness, if not of distinct images. Prof. Agassiz's observations of the 

 muscular system of Medusae were made on them when in a state of 

 contraction immediately after death. During life the movements are 

 too active to permit close examination. Proceedings of the Boston 

 Natural History Society, Jan. 



ANALOGY BETWEEN THE MODES OF REPRODUCTION IN PLANTS, 

 AND THE ALTERNATION OF GENERATION OBSERVED IN SOME 

 RADIATA. 



THE very remarkable fact, that a Polyp and a Medusa may be in 

 some instances different states of one and the same species, has been 

 well established of late by the researches of Sars, Dalyell, Steen- 

 strup and others ; aud recent observations have been made on the sub- 

 ject by Professor Agassiz. The alternations are as follows : 



1. The Medusa produces eggs. 2. The eggs, after passing through 

 an infusorial state, fix themselves and become Polyps, like Corynse, 

 Tubulariee, or Campanularise. 3. The Polyp produces a kind of bud, 

 that finally drops off and becomes a Medusa. Thus the egg of a Me- 

 dusa in such cases does not produce a Medusa, except after going 

 through the intermediate state of a Polyp. Or if we commence with 

 the Polyp, the series is, 1. The Polyp produces bulbs that become 

 Medusae. 2. The Medusa produces eggs. 3. The eggs produce 

 Polyps. 



This is what is called by Steenstrup " alternation of generations," 

 and he considers the earlier generation as preparing the way for the 

 later. It certainly seems a most mysterious process, a parent pro- 

 ducing eggs which afford a progeny w r holly different in form ; even so 

 different that naturalists have arranged the progeny in another grand 

 division of the Radiata, and the progeny afterward, by a species of 

 budding, repeating the form of the original parent. 



Yet, although seemingly so mysterious, is not this mode of develop- 

 ment common in the vegetable kingdom? Is it not the prevalent pro- 

 cess in the plants of our gardens and fields, with which we are all fa- 

 miliar? It is well known to us that in most plants, our trees and 

 shrubs, for example, growth from the seed brings out a bud of leaves ; 

 from this bud, after elongation, other leaf-buds are often developed, each 

 consisting, like the first, of a number of leaves. It is an admitted fact, 

 that each of those buds is a proper individual plant, and that those con- 

 stituting a tree are as distinct and independent as the several Polyps of 

 a compound zoophyte ; and that the tree, therefore, is as much a com- 



