THE CAMPANULAEID^ AND THE BONNEVIELLID^. 



15 



gonophores, or, as AUman calls them, "phanerocodonic gonophores." The sessile gonophore or 

 sporosac is homologically a degraded medusa and, in some cases at least, it is possible to trace 

 in such a gonophore the various layers of the medusa in their proper sequence and relation^ 



While the medusa is structurally a sexual person of the colony, it has an additional func- 

 tion not subserved by the fixed gonophore — ^i. e., that of a "seed can-ier" or distributer of the 

 sexual elements. On this accomit Allman has given the name "planoblast" or "wandering 

 bud" to the medusa.^ This author has done more, perhaps, than any other to elucidate the 

 morphology and homological significance of the various structures fotmd in the Hydroids. 



The typical medusa is composed of a gelatinous bell which may vary in shape from a nearly 

 flat disk (fig. 62) to an almost complete sphere (fig. 64). From the center of the subumbrellar 



medusjE of the campanttlarib^. 



Yig. 62.— Obelia flabellata . 



Fig. C3. — Otocyst of Orthopyxis campanularia. (After von Lendenfeld.) ft, velum; o, otolith. 



Fig. 64. — Agastra mira (= Orthopyxis caiiculata), (After Mayer.) 



surface there usually hangs the manubrium which, in the CampantdaridEe, is very simple in 

 form, often being a knob-like body with the opening or mouth at its distal end. This mouth 

 commimicates with the gastric cavity centrally located in the bell. The four corners of the 

 gastric cavity are drawn out into the four equidistant radial canals which pass down immedi- 

 ately above the subumbrellar surface to the ring canal which encircles the edge of the bell. 

 The marginal tentacles are found in the medusae of all campanularians except in the genus 

 Orihojjyxis. 



All of the medusse of the Campanularid.'e belong to the order LeptomedusfB of Haeckel, 

 defijied as follows by Mayer: ^ 



HydromedustE which arise by alternation of generation from campanularian hydroids. Tlie gonads are developed 

 upon the radial canals. When present, the Hthocysts are of ectodermal origin. 



This author, however, uses the term "campanularian hydroid" so as to include a number 

 of forms, e. g., Campanulinh)^, which do not come under the definition of Campanularidse 

 as used in the present work. All of the medusae produced by the Campanularidse as here 

 understood would come under the family Eucopidas as used by Mjayer, who defines this family 

 as "Leptomedusse with lithocysts and with less than eight radial canals upon which the gonads 

 are developed." * 



^ See page 30 of the first part of this work for a discussion of the homology of the corresponding layers of a gono- 

 phore and medusa. 



^ Challenger Reports, Hydroida, part 2, 1888, p. xxvi. 



^ Medusa of the World, vol. 1, th^ Hydromedusse, 1910, p. 196. 



*Idem, vol. 2, the Hydromedusae, 1910, p. 231. 



