iv PHYLUM CCELENTERATA 213 



tudinal muscular fibres, which, as we shall see (p. 216), are of meso- 

 dermal origin, and which serve to retract the tentacle into its sheath. 



Delicate muscle-fibres lie beneath the external epithelium and 

 beneath the epithelium of the canal-system, and also traverse 

 the mesogloea in various directions. The feeble development 

 of the muscular system is, of course, correlated with the fact that 

 the swimming-plates are the main organs of progression, the 

 Ctenophora differing from all other Ccelenterata in retaining cilia 

 as locomotory organs throughout life. 



A further striking difference between our present type and the 

 Coelenterata previously studied is the absence, in Hormiphora, of 

 stinging-capsules. The place of these structures is taken, in a 

 sense, by the peculiar adhesive-cells with which the branches of 

 the tentacles are covered. An adhesive-cell (Fig. 162, ad. c.) has 

 a convex surface, produced into small papillae, 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 



+rvrr a\xra\r K^r fhp FIG. 163. Hormiphora plumosa, Sense-organ, ft. bell: 

 Vd V U J c. p. ciliated plate ; c. gr. ciliated groove ; ex. p. excretory 



Struggles of the Pore ; I. lithites ; p. pi. polar plate ; sp. spring. (Modified 



from Chun.) 



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. 163, 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 

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

 lithites of 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 bell (&.), 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 



