110 



COMPARATIVE ANATOMY. 



they do not lie in the free cavity of the vesicle they cannot be 

 definitely regarded as similar to the auditory vesicles of other low 

 animals; at the same time, it is not possible to give them exactly 

 any other signification. It is clear that they represent sensory 

 organs from their intimate connection with the nerve-ring, for a 

 double fibrous band arises from the ganglion, which is placed below 

 each marginal vesicle, and surrounds the vesicle ; after uniting 

 with it, the fibres pass into the spherical mass of cells, which 

 contain the concretion (Geryonidas) . This marginal vesicle is 

 most common among the Eucopidte, Trachynenriche, Geryonida*, 

 and /Eginida). 



In Cunina crystals are present, so that its marginal vesicles form 

 an intermediate step towards the similar structures in the Discophora. 

 The marginal vesicle in the Discophora is always stalked (Fig. 40, 

 A B b), and lies in a fissure, or a niche-like depression of the edge 

 of the disc, covered with a lamellar umbrella-like process of it. 

 A cavity (ampulla) forms a large part of the marginal body (d), 

 and is connected with the gastrovascular system by means of a 

 canal, which passes into the stalk (c) ; attached to this ampulla, and 

 occupying the free edge of this marginal body, is a vesicle (e) 

 filled with crystals, and resembling the similar one in the .ZEginida} 

 (Cunina). The most important difference between the two is 

 therefore the absence in Cunina of the ampulla formed by the 

 gastrovascular system. 



Organs of another kind also are found in the Hydromedusas. 

 They appear to be in a relation of mutual exclusion to the marginal 

 vesicles, as they appear in those families only (Oceanida)) which 

 have no vesicle. The first indication of them is the appearance 

 of pigment spots on the base of the tentacle, which, as a rule, have 

 no refractive media ; in other cases, on the contrary, they are pro- 

 vided with structures which call 

 to mind the crystalline cones 

 of other lower animals. In the 

 Discophora these ocelli are com- 

 bined with the already-men- 

 tioned marginal bodies ; they 

 sometimes consist of pigment 

 only, while in other forms the 

 pigment appears as the invest- 

 ment of a highly refractive body 

 (Fig. 40, Bg). ' 



In the Ctenophora, also, there 

 are special sensory organs. The 

 most important is a vesicular 

 structure, which is attached to 

 the aboral pole of the body, and 

 contains solid concretions, like 

 the otoliths in the auditory vesicles of other lower animals. The 



Fig. 40. Marginal body of Aci'aspcdoho 

 Medusae: A Of Pelagia noctiluca; B 

 Of Charybdea rnarsupialis. «Thc free 

 part of the marginal body placed between 

 the marginal notches in the disc, b Stalk, 

 c Canal in it. d Ampulla, e Crystallino 

 saccule. / Pigment, g Lens-like body. 



functional importance of this 



organ 



is, however, not yet exactly 



