iv PHYLUM CCELENTERATA 201 



The tentacles in Zoantharia are usually very numerous, and in 

 nearly all cases have the form of simple glove-finger-like out- 

 pushings of the disc. In Edwardsia, however, they may be 

 reduced to sixteen, and in some genera of Sea-anemones they are 

 branched. In the Antipatharia (Fig. 150) they vary in number 

 from six to twenty-four. When more than six are present, six 

 of them are larger than the others. 



FIG. 150. Antipathes ternatensis, portion of a branch, showing three zooids and the horny 

 axis beset with spines. (From the Cambridge Natural History, after Schultze.) 



In the Alcyonaria, on the other hand, the tentacles, like the 

 mesenteries, are eight in number and are always pinnate, i.e. 

 slightly flattened and with a row of small branchlets along 

 each edge (Fig. 144). Many Actiniaria have the tentacles 

 perforated at the tip (Fig. 138, 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. 138, A, and Fig. 157, 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. 143, 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 

 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. 151), 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 the lateral parts each 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 



