216 ZOOLOGY sect. 



are covered. An adhesive-cell (Fig. 161, ad. c.) has a convex surface, 

 produced into small papilhe, which readily adheres to any object 

 with which it comes in contact and is with difficulty separated. 

 In the interior of the cell is a spirally coiled filament, the 

 delicate inner end of which can be traced to the muscular axis of 

 the tentacular branch. These spiral threads act as springs^ and 

 tend to prevent the adhesive-cells being torn away by the 

 struggles of the captured prey. 



Both the central nervous system and the principal sense- 

 organ are represented by a peculiar apparatus situated, as already 

 mentioned, at the aboral pole. In this' region is a shallow depres- 

 sion (Fig. 162, c. p.) lined by ciliated epithelium and produced in 

 the transverse plane into two narrow ciliated areas, the 'polar 

 plates (p. pi.). From the depression arise four equidistant groups 

 of very large S-shaped cilia (sp.), united to form as many springs (*p.), 

 which support a mass of calcareous particles (/.), like the lithites of 



Fig. I i-.-Hormiphora plumosa, Sense-organ : b. bell ; c. p. ciliated plate ; c. gr. ciliated 

 groove ; ex. p. excretory pore ; I. lithites ; p. pi, polar plate ; sp. spring. (Modified from Chun.) 



Hydrozoa and Scyphozoa. From each spring a ciliated groove (c. gr). 

 proceeds outwards, bifurcates, and passes to the two swimming- 

 plates of the corresponding quadrant. The lithitic mass, with 

 its springs, is enclosed in a transparent case or tell (b.), formed of 

 coalesced cilia. It appears that the whole apparatus acts as a 

 kind of steering-gear, or apparatus for the maintenance of equili- 

 brium. Any inclination of the long axis must cause the calcareous 

 mass to bear more heavily upon one or other of the springs : the 

 stimulus appears to be transmitted by the corresponding ciliated 

 groove to a swimming-plate, and results in a vigorous movement 

 of the combs. Thus the sensory pit acts as a central nervous 

 system, and the ciliated grooves as nerves. A sub-epithelial 

 plexus of nerve-fibres with nerve-cells extends all over the surface 

 of the body. 



Reproductive Organs. — The animal is hermaphrodite, the 

 organs of both sexes being found in the same individual. The 

 gonads are developed in the meridional canals (Fig. 159, B), each of 

 which has an ovary {ovy.) extending along the whole length of one 

 side, a spermary (spy.) along the whole length of the opposite side. 



