ACTrNOZOA. 



223 



Fm ' ie *- Gr P <* nomatocysta at th* end 



of the tentacle of a Scyphist ma 



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

 llast, 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. 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 



which they stand FIG. 169. Longitudinal section through the nerve ring of Ckaryldea. 

 in connection ^ z ' Sense cells in the ectoderm; Gz, ganglion cells; Iff, 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 polypf). 



Polyps with cesophageal tube and mesentcric folds, with interim! 

 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 pbysiologischen Kenntm'ss der Kornllenthiere im 

 Allgemeinen nnd besonders des rotlicn Mccrcs." Ehrenberg. " Uber die Natur 

 und Bildung der Korallenbanke," Alili. dcr Berl. ATtad, 1832. Ch. Darwin, 



