Polypes (Anthozoa). 



63 



type is the Sycon raphanus, of which a variety pecuhar to the 

 Aquarium grows thickly on the walls of its tanks (fig. 159). 



Although the sponges are, in the adult condition, firmly fixed 

 to the substratum, yet in the youngest stages, 

 as mentioned above, they swim freely around as 

 so-called Larvae and chose their settling place 

 from which they are later unable to move. In 

 this manner the calcareous sponge just spoken 

 of settles in the Aquarium and, as its wide dis- 

 tribution shows, flourishes. The other sponges 

 exhibited are fished for in the Bay and remain 

 alive for a long time and some even increase 

 by a curious process. Thus after some months 

 the tissues of Axinella degenerate for the most 

 part, while those portions which have remained 

 healthy fall to the ground and grow out to 

 new bushes. Another sponge, Chondrosia, be- 

 comes blown out to a transparent bladder by 

 gas which is formed within the body. Eventually 

 it bursts and out of the pieces arise new colonies. 



Fig. 159- 



Some specimens of 



Syioti raphanus, 



attached to a piece 



of rock at the 



left hand. 



Polypes (Anthozoa). 



If it be difficult to the lay mind to apply the term Sponge to 

 organisms, which in a living condition are not at all of a spongy 

 nature, it will be found equally difficult to picture as Corals any- 

 thing else than the beautiful red and white branches which are 

 displayed as ornaments on writing-desks and chimney-pieces. 

 And yet these branches are not really the animals themselves, but 

 only the framework which they have built themselves and in 

 which they live imbedded, in hundreds or thousands, side by side 

 or one above the other. Of the folypes, the animals which build 

 up the corals, the best conception may be gained by examining the 



Sea-Anemones (Actiniae). 



These animals exhibit a cylindrical body, attached by an 

 adhesive disk to some fixed object and bearing at its free end 

 numerous very mobile tentacles. These encircle an aperture 

 which serves both as mouth and as anus (fig. 49); it leads into 

 a capacious stomach in which the food is digested. The soft and 

 apparently unprotected polype is really very well armed. Many 

 parts of the body, but especially the tentacles which serve to 

 catch its prey, are provided with numerous microscopic vesicles, 

 the so-called stinging-cells, each containing an acid liquid and a 

 spirally coiled thread. When the animal comes in contact with 

 its enemies or its prey thousands of these stinging-cells burst, 



