04 PROF. ALLMAN ON STEPHANOSCYPHUS MIRABILIS. 



General Remarks — Homology . 

 From the account now given of Stephanoscyphus mirabilis, it is evident that we have in 

 this remarkable animal a true hydrozoal structure ; but while its apparent form and habit 

 are those of a hydroid trophosome, its organization is, in its essential features, that of a 

 hydroid planoblast or medusa. If we imagine a campanularian hydroid in which the 

 hydranths have been replaced by peculiarly modified medusas, capable, as in the hydranth, 

 of exsertion beyond the hydrothecse, and of retraction within them, we shall form very 

 nearly a true conception of Stephanoscyphus. The wide thin- walled chamber into which 

 the terminal orifice opens would thus correspond to the umbrella of a medusa ; and the 

 terminal orifice itself, though neither lithocysts nor ocelli appear to be developed 

 on its margin, would be homologous, not with the mouth of a hydranth, but with the 

 codonostome or umbrella-aperture of a medusa ; while the tentacular crown would admit 

 of a direct comparison, not with the tentacular crown of a hydranth, but with the mar- 

 ginal tentacles of a medusa. The true mouth would thus have to be sought for at the 

 point where the wide distal cavity passes into the narrow proximal cavity which occupies 

 the axis of the stem. Then there are the four longitudinal canals running symmetrically 

 in the umbrella-walls, and the circular canal surrounding the codonostome, all so many 

 obvious features of medusal organization. It is true that no manubrium appears to be 

 developed, and the bottom of the umbrella-cavity seems to pass uninterruptedly into the 

 true gastric cavity (which is continued through the axis of the stem), neither is there a 

 velum developed at the codonostome, nor do the walls possess the gelatinous consistence 

 and transparency of the typical medusae ; but these special modifications in no degree 

 invalidate the points of homology here insisted on. In the absence of a manubrium we 

 are reminded of the swimming-bells or nectocalices of the Siphonophora, which, though 

 closely resembling in all other respects a true hydroid medusa, and undoubtedly homo- 

 logous with it, are destitute of a manubrium. 



If the views now advocated be correct, the wide cavity into which the terminal orifice 

 opens ought to have no endodermal lining : and this I believe to be really the fact ; for a 

 very careful examination has failed to demonstrate more than a single layer in its walls. 

 On the contrary, the endoderm of the circular canal is very obvious; and though a 

 proper lining membrane cannot be distinctly recognized in the longitudinal canals, this 

 ought not, in an observation rendered difficult by the narrowness of these canals and by 

 the want of transparency of the tissues, to be urged as an argument against its presence. 

 On the other hand, the axile cavity of the stem appears to possess a well-developed endo- 

 derm, and the proper gastric cavity must be regarded as confined to this part. 



The peculiar position assumed by the tentacles in extreme retraction is very different 

 from the condition of these parts in the retracted state of the hydranth in a hydroid 

 trophosome, and quite corresponds with that which they present in the greater number 

 of young medusiform planoblasts, in which before the planoblast becomes free the 

 marginal tentacles are thrown back, and lie entirely within the cavity of the umbrella. 



1 nave elsewhere * endeavoured to show that the radiating canals of a medusa are 

 strictly homologous with the tentacles of a hydranth, and that, if the latter were con- 



* Monograph of Gymnoblastic Hydroids. 



