ACTItfOZOA. 



223 



principally of the ectoderm, but also of the entoderm. Each cnido- 

 blast, from the contents of which a nematocyst is developed, possesses 

 a fine superficial plasmatic process (cnidocil), which is probably very 

 sensitive to mechanical stimuli, 

 and occasions the bursting of 

 the capsule. 



Very frequently the cnidoblasts 

 are found thickly grouped to- 

 gether at certain places, and form 

 wart-like swellings or batteries 

 (fig. 168). The differentiation of 

 tissues and organs also appears to 

 have i cached a higher stage in 

 the Cnidaria, in comparison with 

 the Porifera, in which cnidoblasts 

 have not hitherto been discovered. L Sense cells, in particular, a: e 

 found in the ectoderm, and these are not seldom grouped together 

 as specific sense organs. Nerve cells and fibres are also present ; 

 the latter often 

 form a deeper 

 layer of fibrous 

 tracts beneath 

 the superficial 

 layer of the ec- 

 toderm, with 



FIG. 168. Group of nematocysts at tlu- end 

 of the tentacle of a Scyphist^ma 



-Stl 



which they Stand FIG. 169. Longitudinal section through the nerve ring of Charybdea. 

 i n connection Sz> Sense cells in the ectoderm ; Gz, ganglion cells ; Nf, nerve 

 fibres ; Stl, supporting lamella ; E, entoderm cells. 



through pro- 

 cesses of the sense cells. Amongst many Medusae (Craspedota and 

 Charybdea) we find a single or double nerve ring near the edge 

 of the disc, while in the Polyps (Actinia), the nerve fibres have a 

 more irregular distribution (fig. 169). 



CLASS I. ANTHOZOA*=ACTINOZOA (Coral polyps). 



Polyps with cKSOpliageal tube and mesenteric folds, with internal 

 generative organs (no medusoid sexual generation), usually with solid 

 mesodermal calcareous skeleton. 



The polyps of the Actinozoa are distinguished from the polyps 



* Ehrenberg, " Beitrage zur physiologischen Kenntniss der Korallenthiere im 

 Allgemeincn und besonders des rothen Meeres." Ehrenberg. " Tiber die Natur 

 und Bildung der Korallcnbanke," Abh. der Berl. Akad, 1832. Ch. Darwin, 



