112 



Goelentera. 



of the mesenteries. These tentacles are hollow evaginations of the 

 body-wall and correspond in position with the spaces between the 

 mesenteries ; they are richly provided with nematocysts. The some- 

 what discoid expanse within the circle, in the centre of which is the 

 mouth, is termed the oral disc, the lower flattened extremity of 

 the body the pedal disc. 



Each mesentery is a fold of endoderm, covering a lamina of 

 mesogleea, which arises from the mesoglsea of the body-wall 

 (^ee Fig. 67). Along the inner edge are cord-like thickenings, 



much coiled and provided with 

 nettle and gland-cells ; these 

 are termed mesenterial 

 filaments; they play an 

 all-important part in digestion ; 

 which, indeed, is probably 

 effected by them alone.* The 

 ova and spermatozoa 

 develop in the mesenteries 

 by the modification of endo- 

 dermal cells; the part of a 

 mesentery in which they occur 

 is thickened, and may be termed 

 ovary or testis. The Anthozoa are usually of separate sexes, 

 only in exceptional cases hermaphrodite. They generally develop 

 hard parts, which will be considered under the different 

 orders. 



The Anthozoa, like other Goelentera, exhibit a radial symmetry, which is,, 

 however, rarely cpmpletely carried out. In transverse section, the stomodaBum 

 is almost always oval, the external mouth slit-like, and thus a median plane 

 is defined. Each end of the oval corresponds with a tentacle ; moreover, the 

 muscles in the mesenteries usually form a thickening on the one side only ; 

 the thickenings may occur on either side, but are always an-anged sym- 

 metrically with regard to the median plane noticed above (see Fig. 67, 

 which shows the arrangement in an Aloyonarian). 



In many of the Zoantharia (at all events, in most of the Actinians), the 

 stomodseum is furnished with two thickly-cihated grooves, corresponding with 

 the two ends of the oval; when the stomodseum is elsewhere pressed together 

 these two grooves remain open, and probably serve to maintain the current of 

 water necessaiy for respiration. In many of the Alcyonaria there is a 

 similar groove, characterised by deep epithelium and long cilia at one side only ; 

 the cihary cun-ent here sets from without inwards, in other regions of the 

 stomodseum in the opposite direction. 



In most corals asexual reproduction occurs by budding 

 or fission. The young animals thus formed separate from the 



Fig. 67. Transverse sections of an Al- 

 oyonarian. Diagrammatic. A through the 

 stomodseum. B lower down, s stomodaeum, 

 li gastric cavity, mit muscle. The ectoderm 

 is represented by an entire hne ; the endoderm 

 dotted ; the mesoglsea shaded. — After v. 

 Koch. 



* From the lower end of the mesenteries in some Actinia, there arise peculiar free 

 threads, similar in structure to the filaments and very rich in thread cells ; they 

 may be shot out through the body- wall, and serve for attack or defence (acontia). 



