iv PHYLUM CCELENTERATA 199 



perforated at the tip (Fig. 139, A, p.) ; and in some species 

 these organs undergo degeneration, being reduced to apertures 

 on the disc, which represent the terminal pores of the vanished 

 tentacles and are called stomidia. 



Many Sea-anemones possess curious organs of offence called 

 acontia (Fig. 139, A, and Fig. 158, ac.). These are long 

 delicate threads springing from the edges of the mesen- 

 teries : they are loaded with nematocysts, and can be protruded 

 through minute apertures in the column, called " port-holes " or 

 cinclides (en.). 



Enteric System. The gullet in the Actiniaria presents some 

 remarkable modifications. It is usually a compressed tube with two 

 siphonoglyphes, but in Zoanthus and some other genera the ventral 

 gullet-groove alone is present (Fig. 144, B), and in Gyractis both 

 grooves are absent, and the tube itself is cylindrical with a circular 

 mouth. The ordinary compressed form of gullet often assumes, in 

 the position of rest, an oo -shaped transverse section, owing to 



FIG. 152. Antipatharia. A, oral face of zooid of Parantipathes. B, oral face of 

 zooid of Schizopathes. (After Delage et Herouard.) 



its walls coming together in the middle and leaving the two ends 

 wide open. In most of the Antipatharia the zooid is drawn out 

 in the direction of the long axis of the branch (Fig. 152), and in 

 some it becomes constricted into three parts (B) which may have the 

 appearance of separate zooids, the central part containing the gullet 

 with the mouth, while each of the lateral parts contains a gonad ; 

 each of these apparent zooids bears two of the six tentacles ; the 

 median one has all six mesenteries attached internally to the gullet ; 

 in each lateral part there is only the outer portion of one of the 

 transverse mesenteries. In such a form as Schizopatkes (Fig. 152, B) 

 there is thus recognisable an arrangement of the parts which might 

 be interpreted as a dimorphism of the zooids, one set the parts 

 containing the mouth and gullet being regarded as gastrozooids, 

 and the others containing the gonads as gonozooids. 



Fixed and Free Forms. A large proportion of Actinozoa are 

 permanently fixed, such, for instance, as most of the Stony Corals, 

 the Sea-fans, Black Corals, &c. Most Sea-anemones are tempo- 

 rarily attached by the base, but are able slowly to change their 



