442 KEVIEWS. 



ascertained that it undergoes a direct metamorphosis from the egg to 

 the perfect Medusa, I would not hesitate to consider it as a member 

 of the order of Hydroids, since it has simple radiating chymiferous 

 tubes, a circular tube, and marginal tentacles closely connected with 

 it, and provided with mere pigment specks upon their base."* 



"With the above statements, in so far as they tend to refute the 

 widely-prevalent error that the affinity between these Acalephs and 

 the fixed Hydroids rests solely on embryological evidence, we desire 

 fully to concur. 



In the Section on Morphology and Nomenclature,! _ Professor 

 Agassiz introduces certain new terms, explanatory of his peculiar 

 views on the Eadiata in general, and the class Acalephae in particular. 

 To the entire body- wall of a radiated animal he applies the designa- 

 tion of spherosome, for " it requires [he says] no formidable stretch of 

 the imagination to reduce any single Polyp, or any Acaleph, or any 

 Echinoderm, to a spheroidal form. Indeed, the sphere is the essential 

 form of all Eadiates, — not the mathematical sphere, but the organic 

 sphere, loaded in different directions, according to the peculiarities of 

 the subordinate groups of this type." " Considering the plan of 

 their structure, we have already seen that there exists in all of them 

 one axis and centre of radiation, around which all their parts are 

 symmetrically arranged in a radiating and concentric order, even 

 though that axis or centre of structure be not the centre of figure or 

 form. At one end of this axis we invariably find the so-called mouth 

 or actinostome, while the opposite end of the alimentary canal may 

 have an excentric position." That side or pole of the sphere at 

 which the actinostome occurs is termed actinal ; the opposite side or 

 pole, abactinal. For the " homological segments," or "identical ele- 

 ments," which together make up the spherosome, the title of sphero- 

 tneres is appropriated. 



The names Planula, Hydra, Scyphostoma, Strolila, JEphyra, and 

 Medusa, taken from Sars and others, are suggested as well suited for 

 distinguishing the principal forms of simple Acalephs, at the several 

 stages of their growth. The term Hydra, after the manner of 

 Dalyell, is also used as a synonym of the common nouns polype and 

 polypite. "When a composite community of Acalephs consists of a 

 number of such Hydrae united by a connecting stem, the whole mass 

 constitutes an Hydrarium. Should bunches of Medusae be budded 

 forth either by any of the Hydrae or their common trunk, each of 

 these groups becomes a Medusarium, and the compound organism 

 which results, including both Hydrae and Medusae, " may be called a 

 Hydro-Medusarium." 



* Pp. 119-20. 



f The distinction between Nomenclature and Terminology, insisted on by Dr. 

 "Whewell and others, is not acknowledged by Professor Agassiz. 



