THELACEROS RHIZOPHOR®. 561 
one tentacle communicates with a radial chamber, with the 
normal tentacles covered by small compound hollow pro- 
tuberances. The accessory tentacles rudimentary. 
Thelaceros. 
Shallow water. 
These two families are closely allied. The centripetal dislo- 
cation of the tentacles, which is so well marked in Coralli- 
morphus, is only indicated in Thelaceros. In both there is a 
striking arrangement of some of the tentacles in groups of 
three. In both the pedal disc is small compared with the oral 
dise, and in both there is an indistinctness or absence of sipho- 
noglyphes. In both the polyps are of an unusually firm con- 
sistency. The large size and the strong development of the 
longitudinal muscles in Thelaceros contrast with the small size 
and weak musculation of Corallimorphus. But the difference 
in conditions of existence between the crowded life of a man- 
grove swamp (Hickson) and the monotonous repose of deep 
water may well be connected with structural differences. The 
Thelaceride and Corallimorphide are probably not very far 
removed from a primitive Hexactinian stock. This stock was 
probably devoid of sphincter or acontia, and it had a weak 
musculation and pedal disc very small or absent. The stock 
would, in fact, be not very far removed from the Anthro- 
morphide of Hertwig. The Corallimorphidze going to deep 
water required and developed no strong muscular system. 
The conditions of existence for the ancestral Thelaceridz 
made protection an advantage or a necessity to the ancestral 
Thelaceride. The ordinary Hexactinians are protected by an 
occlusion of the oral disc over the tentacles ; the Thelaceride 
by a sudden vertical contraction withdraw themselves into the 
mud, a continual selection having favoured those with strong 
longitudinal muscles. The oral disc remained uncontracted, 
and in correlation with this the tentacles became peculiarly 
modified. Not improbably another method of protection, the 
increasing the rigidity of the polyp—a rigidity incipient in 
Thelaceros and Corallimorphus—by the disposition of cal- 
