GNIDABIA— SENSORY ORGANS 



97 



interradial slits in the bell -shaped roof. They divide into 8 

 adradial ciliated bands, which run along the aboral surface of the 

 Gtenophoran body towards the upper end of the rows of swimming 

 plates. Closely bordering on the sensory body are two ciliated 

 epithelial bands, the so-called pole plates (Fig. 74, pp), whose edge is 

 thickened. They lie in the median plane of the body ; at the point 

 where they come in contact with the sensory body are found the two 

 above-mentioned median slits through the bell covering that body. 



Organs of sight. — Simple organs of sight occur principally as 

 pigment spots in such Leptomedusce as possess no marginal vesicles 



Fig. 74.— Aboral pole of the hody of 

 Gallianira, after Hertwlg. ws, Ciliated 

 bands ; /, springs carrying tbe mass of 

 otoliths (o) ; sic, sensory body ; pp, pole 

 plates ; to, openings of the 4 branches of 

 the aboral vessel or gastro-canal. 



Pig. 75.— Halves of two sections through the 

 sensory body of Callianira, after Hertwlg. A 

 passes through one of the 4 springs (/) which 

 carry the otolith mass (o) ; d, roof of the sensory 

 pit ; se, sensory epithelium of the sensory body ; 

 p, pigment. 



{Ocellata), and in AntJiomedusce. They consist of pigment cells and 

 sensory cells, to which, in some cases, a cuticular thickening is added 

 as lens. Such a lens is less often wanting in the visual organs of the 

 Acraspeda (Fig. 73, oc, I), whose structure is occasionally considerably 

 complicated ; they form part of the marginal bodies. In the Cubo- 

 meduscB a vitreous body and a retina are developed between the lens 

 and the pigment cup. In Gharyhdea there are compound eyes ; each 

 marginal body bears 2 large unpaired and 4 small paired eyes. 

 Eye spots, sometimes with lenses, are found at the tips of the feelers 

 in some Siphonophma. In the Gtenophora there are, as has already 

 been mentioned, pigment spots in the sensory cushion of the sensory 

 body. 



VIII. Supporting Organs, Protective Organs, Skeleton. 



The formations belonging to this category, which are very hetero- 

 geneous, can at once be divided into two principal groups, according to 

 their ectodermal or mesodermal origin. 



VOL. I H 



