24 ON THE AFFINITIES OF THE FAMILY OF THE MEDUSAE 



mined in two Avays, either — i, by tracing back the course of develop- 

 ment of the two until we arrive by similar stages at the same point ;, 

 or, 2, by interpolating between the two a series of forms derived from 

 other animals allied to both, the difference between each term of the 

 series being such only as can be accounted for by the laws of growth. 

 The latter method is that which has been generally employed under 

 the name of Comparative Anatomy, the former being hardly applicable 

 to any but the lower classes of animals. Both methods may be made 

 use of in investigating the homologies of the Medusas.^ 



58. A complete identity of structure connects the "foundation 

 membranes " of the Medusae with the corresponding organs in the 

 rest of the series ; and it is curious to remark, that throughout, the 

 outer and inner membranes appear to bear the same physiological 

 relation to one another as do the serous and mucous layers of the 

 germ ; the outer becoming developed into the muscular system and 

 giving rise to the organs of offence and defence ; the inner, on the 

 other hand, appearing to be more closely subservient to the purposes 

 of nutrition and generation. 



59. The structure of the stomach in the Medusae is in general 

 identical with that of the same organ in the rest of the series. The 

 Rhizostomidae offer an apparent difficulty, but it appears to me that 

 the marginal folds in them answer to the stomachal membrane of the 

 Monostome Medusae ; the apertures to the inner orifice of their 

 stomach, and the common canal to their " common cavity." Just as 

 in a polygastric Diphyes the common tube answers to the chamber 

 into which the stomach of a monogastric Diphyes opens ; and in 

 CepJiea Wagneri (Will) these resemblances are still more striking. 

 He says that each cotyledon " has at its apex a small round opening, 

 the mouth, which leads to an ovate cavity, occupying the whole 

 interior of the cotyledon. I consider this as the proper digestive or 

 stomachal cavity, and believe that the cotyledons have the same 

 relation to the vessels as the so-called suckers {Savgrohrcn) of the 

 Diphyda; to the common tube (Saftrohre)." - 



60. The disc of a Medusa is represented by the natatorial ora-an 

 among the Diphyds and Physophoridse. Take for instance the disc 

 of Ocea?iia or Cytceis. It is here a more or less bell-shaped body, 

 traversed by radiating canals, lined by a distinct membrane, united 



' The above definitions may be thouglit needless and even trite, but the estabhshment of 

 affinities among animals has been so often a mere exercise of the imagination, that I ma)- 

 be pardoned for pointing out the guiding principles which I have followed, and by which I 

 would wish to be judged. 



^ Horfe Tergutina;, p. 60. 



