16 AMERICAN HYDROIDS. 



Most of the medusse of the Campanulariclse belong to the subfamily of the Eucopidae, the 

 Obelinse, characterized by the possession of eight adradial lithocysts and a manubrium without 

 a peduncle. JMedusse of but tliree genera of this subfamily have been definitely identified 

 \vith hydroid colonies. These are OrtTiopyxis Agassiz, Agastra Hartlaub, and Ohelia Peron 

 and Lesueur. In the present work Agastra is regarded as generically identical with Orthoijyxis on 

 accomit of the very great similarity of the hydroid stocks, and Ohelia is retained. 



One genus of Campanularidte produces medusae belongiug to Mayer's second subfamily, 

 the Phialinfe, characterized by having more or less than eight lithocysts and a manubrium 

 with a well-developed peduncle. The well-known genus Olytia belongs to this group, its medusae 

 conforming to the diagnosis given above. 



The homology existing between the medusa and the ordinary gonophore has already been 

 referred to. The gonophore itself is generally regarded as a sexual offshoot from the blasto- 

 style and the latter is generally looked upon as homologous with a* hydranth without tentacles 

 and ordinarily without a mouth. 



Various authors have also drawn attention to the homologies which can be shown to e^dst 

 between the medusa and the hydranth. If a medusa of simple form be inverted and compared 

 with a hydi-anth it is quite evident that most of the parts of the one are strictly comparable 

 with similar parts of the other. For instance: 



The manubriimi of the medusa is equivalent to the proboscis of the hydranth. 



The gastric cavity of the medusa is homologous with the body cavity of the hydranth. 



The bell of the medusa is comparable to the body of the hydranth in which the stutzla- 

 mella is immensely thickened to form the "jelly" of the medusa. 



The tentacles (marginal) of the medusa are homologous with the tentacles of the hydranth. 



There are two structures in the campanularian medusae that have no homologues in 

 the hydranth — i. e., the velum and the lithocysts. The former is an organ of locomotion that 

 is not called for in the sessile hydranth, and the latter is a sense organ concerned prunarily with 

 orientation, a function not of great value in a fixed form. 



The lithocysts (fig. 63) are foimd in all medusae produced by the Campanularidae, and 

 constitute the first specialised sense organs that we have thus far encountered in our study 

 of the Hydroida. They thus deserve more than a passing mention here. 



These structures are ectodermal in origin and eight in number in all Campanularidae with 

 known hydroid stocks, except in the genus Olytia where there are 16 lithocysts in the adult 

 medusa. In all Leptomedus* they arise as folds of the velum near the margin of the bell and 

 are therefore ectodermal. In Ohelia they lie imder the bases of the tentacles where the latter 

 join the margin. In Olytia they are placed on the margin between the tentacle bases. In 

 OrtTiopyxis they are eight in number equally spaced around the margin or, in 0. caliculata, they 

 are on the margm near the radial canals. 



Within the globular lithocysts the otoliths (fig. 63, o) appear as calcareous concretions 

 looking like glassy bodies, in many cases single, as in several species of Ohelia; while in other 

 cases, e. g., Tiaropsis, there are several of these bodies, in some cases arranged in a crescent, as 

 represented by Mayer.^ The ectodermal cells forming the inner wall of the lithocyst are supposed 

 to be sensory. Von Lendenfeld foimd a number of columnar epithelial sense cells lining the 

 otocyst on the side nearest the velum in OrtTiopyxis campanulana (fig. 63, Ti) and a lenticular 

 otolith with a diameter of about half that of the cavity of the otocyst.^ The otoliths are said 

 to be suspended in a fluid and the movements or alterations in position of the medusa cause 

 a corresponding movement of the suspended otoliths, bringing them m contact vAt\\ the sense 

 cells and thus serving a function in the orientation of the organism. 



The radial canals are normally four in number m aU medusae that are known to be pro- 

 duced by campanularian colonies, and the gonads or ovaries are always foimd (when developed 



' Medusa of the World, vol. 2, the Hydromedusae, 1910, pi. 32, fig. 9. 

 - ijber Coelenteraten der Sudsee, part 4, plate 32, fig. 35. 



