166 LECTURE IX. 



performed in the cavity of the proboscis. At the fundus of this 

 cavity Will saw in Geryonia pellucida four small obtuse prominences, 

 each of which presented a small aperture leading to the system of 

 the ciliated chylaqueous canals. In Thaumantias leucostylus he 

 found a distinct ciliated cavity separated from the stomach at its base 

 and from which the chylaqueous vessels sprang. Prof. E. Forbes also 

 found either a well-defined cavity at the base of the stomach or 

 an indication of such a cavity, from which the chylaqueous vessels 

 were continued.* This cavity and system of vessels is homologous 

 with that to which I have given the name "chylaqueous" in the 

 Anthozoic and Sertularian polypes. 



A progressive simplification of the appendages of the mouth may 

 be traced from the Cyangea downwards in the small bare-eyed^ and 

 probably larval forms of Medusse : in Geryonia and Oceania the 

 lips are produced into fimbriated lobes ; in Circe and many species of 

 Thaumantias into simple lobes ; in Sarsia the lip is a thickened ring 

 round the orifice of the tubular digestive cavity.f 



In all Pulmogrades the inner or lining membrane of the digestive 

 and nutritive cavities and canals is soft and cellular, and lined by a 

 ciliated epithelium : it is applied either to the gelatinous basis of the 

 disc, or directly to the integument, or to the walls of the generative 

 cavities (e e,jfig. 76.). The gelatinous basis of the disc in Rliizostoma 

 consists of " an apparently homogeneous substance containing a mul- 

 titude of delicate fibres interlacing in every direction, in the meshes of 

 which lie scattered nucleiform bodies :" \ its upper surface is covered 

 by a tessellated epithelium of polygonal nucleated cells ; on the lower 

 surface the epithelium " is replaced by a layer of parallel muscular 

 fibres."§ The integument contains many cells analogous to the 

 thread-cells in the Hydrozoa and Anthozoa, and to which Prof. 

 Wagner has ascribed the function of urticating, calling them " nettle- 

 cells" (nessel-organe). || These cells are usually of an oval form and 

 contain a long filament, which when retracted is spirally disposed, 

 but darts out, often to considerable length, when the skin is touched. 

 They are very numerous in the strongly urticating Pelagia noctiluca : 

 in the feebly urticating Oceania they are principally located in the 

 marginal tentacles ; and Ehrenberg, who detected them in the 

 stinging tentacles of the Cyancea capillata ^, could not find them in 



* CXLVIL p. 4. t CXLVIL p. 8 % CXLV. p. 415. 



§ CXLV. p. 416. II CXLin. p. 38. 



^ Prof. E. Porbes states that only a small minority of the medusae of our seas 

 are stingers, and amongst these the Cyanaa capillata is a formidable one, and " the 

 terror of tender-skinned bathers. With his broad tawny, festooned, and scalloped 

 disc, often a full foot or more across, it flaps its way through the yielding waters 



