410 REVIEWS. 



Professor Agassiz lias not yet finally decided, but he entertains little 

 doubt that the large group of Tabulata ought no longer to be kept 

 apart from the Hydroids proper. This view, apparently so heterodox, 

 is, according to Professor Agassiz, sufficiently sustained by the results 

 of some recent observations which he has made on the soft parts of 

 the living Millepora. The details of these researches have not yet 

 been communicated at length, so that the important enquiry which 

 they suggest to the systematic zoologist cannot now be more than 

 referred to. We may add, however, that the figures of the animal 

 of Millepora alcicomis, which Professor Agassiz has here given, less 

 closely resemble Actinoid polypes than the polypites of Hydr actinia > 

 to which genus he deems the Tabulata to be most intimately 

 related. 



In accordance with recent investigations, Professor Agassiz does 

 not dispute the possible existence of a group of Medusid forms, 

 distinct from those Cryptocarpae which have been shown to be but 

 the free reproductive bodies of the Hydroida. But to the systematist 

 whose mind has been fully made up on the fundamental principles of 

 his science, the existence of such a group makes comparatively little 

 difference, howsoever significant it may justly seem to the student of 

 animal development. 



" Since the free Medusae known to originate from Hydroids all 

 belong to the type of the Discophorcs Cryptocarpae of Eschscholtz, 

 the Gymnopht/ialmata of Forbes, or Craspedota of Gregenbaur, there 

 is presumptive evidence that the final investigation of the true 

 affinities of these Medusae will lead to a natural association of all 

 those which are really and closely related to one another, to the ex- 

 clusion of the possible foreign admixtures now left in this group, 

 and that such a natural group will in the end embrace all the 

 Medusae originating from Hydroids. It is also possible, however, 

 that such a natural group of Medusae may embrace genera under- 

 going a direct metamorphosis from the egg to the perfect Medusa 

 without intervening Hydra stock, as we already know that there are 

 higher Discophorae, such as Pelagia, which reproduce themselves 

 without passing through the Strobila state. But this would not 

 alter the case of the affinity of such Medusae : it would only show 

 that the natural group to which they belong exhibits a wider range 

 in its modes of development. The systematic position of any 

 Medusa must be determined by an investigation of its special 

 structure, and if there are any Medusae, not arising from Hydroids, 

 but growing up directly from eggs to their permanent form, and 

 presenting the same special structure as those that arise from 

 Hydroids, there is no reason why they should be separated. Upon 

 this view we shall hereafter consider the affinities of the iEquoridae, 

 the mode of development of which is not yet fully ascertained, and 



