84 



MORPHOLOGICAL DEVELOPMENT. 



proliferoiisly-produced fronds, after growing to certain sizes 

 and developing rootlets, detacli ttiemselves from their parent- 

 fronds ; so among these animals, separation of the young 

 ones from the bodies of their parents, ensues when they have 

 acquired tolerably complete organizations. 



There is reason to think that the parallel holds still fur- 

 ther. Withia the limits of the Jungermanniacea;, we found 

 that while some genera exhibit this discontinuous develop- 

 ment, other genera exhibit a development that is similar to 

 it in all essential respects, save that it is continuous. And 

 here within the limits of the Hydrozoa, we find, along with 

 this genus in which the gemraiparous individuals are pre- 

 sently cast off, other genera in which they are not cast off, but 

 form a permanent aggregate of the third order. Figs. 149 

 and 150, exemplify these compound Hydrozoor—one of them 

 showing this mode of growth so carried out as to produce a 

 single axis ; and the other showing how, by repetitions of 

 the process, lateral axes are produced. Integrations character- 

 izing certain higher genera of the Hydrozoa, which swim or 

 float instead of being fixed, are indicated by Figs. 151 and 

 152 : the first of them representing the type of a group in 

 which the polypes growing from an 

 axis, or coenosarc, are drawn through the 

 water by the rhythmical contractions ' 

 of the organs from which they hang ; 

 and the second of them representing 

 a Physalia the component" polypes 

 of which are united into a cluster, 

 attached to an air-vessel. It should 

 be added that in the Rhizostomes, the 

 integration is carried so far, that the 

 individualities of the polypes are al- 

 most lost in that of the aggregate 

 they form. 



A parallel series of illustrations 

 might be dra\*Ti from that second di- 



