5°4 



CCELENTERA TES. 



may chance to come within reach. It is interesting to note that a Palythoa 

 closely related to the Japanese form occurs in the Adriatic, and is also attached to 

 sponges, scarcely a single specimen of the sponge in question being found without 

 its polyp guest. The larvae hatched from the eggs of the Palythoa evidently 

 perish unless they meet with one of these sponges ; but the manner in which they 

 find and recognise their particular host is quite unknown. Other species of 

 Palythoa found on the American coast settle on the shells inhabited by hermit- 

 crabs, covering the shell as an uninterrupted mass several lines thick, and the 

 individual polyps rising to about an equal height above 

 the general mass. The shell becomes disintegrated beneath 

 this cover, and the polyp-stock then remains as the only 

 covering to the crab. In this case there is mutual 

 advantage, for the crab is covered and protected by the 

 polyp-stock, while the polyp profits by its wanderings and 

 enjoys constant change of water and new fields for food. 



An extraordinary form, very nearly allied to Zoantharia, 

 has been described under the name of Polypariumn 

 ambulans, and is found in the strait dividing the island 

 of Mindanao from that of Billiton. It consists of a colony 

 three inches' long and six wide, flattened from above down- 

 ward, and therefore more or less ribbon - like ; and the 

 anterior cannot be distinguished from the posterior end. 

 The upper surface of the colony is covered with peculiar 

 polyps shaped like chimneys, the base being much wider 

 than the top, which carries a round aperture. Each polyp 

 is extremely minute, and has no tentacles. They stand 

 in irregular transverse rows of from five to eight, differing 

 in age and therefore in size ; the lower side, on which the 



These 



Palythoa axinellce (somewhat 

 less than nat. size). 



True Corals. 



The small dried-up polyps are colony rests, being beset with protuberant suckers. 



seen adhering in groups to -, ,.» i • • , , . -,. , 



the branching sponge. aiso diner much m size, but stand m regular rows divided 



by furrows, and serve for attaching the colony, and also 

 enable it to creep. The colony can even be seen slowly climbing up and down small 

 stones. The polyps have no septa in the digestive cavity, the inner side being quite 

 smooth ; and the lower end of each is not closed, but communicates with a large 

 cavity running along the whole colony, and divided at regular intervals by partitions. 

 From the foregoing observations it will be seen that in the soft 

 division of the Hexactinia, or six-rayed anemones, there are both 

 single individuals and colonies of individuals joined together to form stocks ; and 

 there is also the same diversity in the skeleton-producing division — the corals 

 proper, where we have both single individuals and stocks. Whereas however, in 

 the soft division, the simple individuals are the more numerous and the colonies 

 comparatively rare, among the corals the opposite is the case, the colony-forming 

 types presenting almost innumerable varieties. This is not difficult to understand, 

 since the soft anemones cannot well form complicated colonies, whereas the skeleton- 

 forming polyps, by combining their skeletons, can build complicated structures, in 

 order to raise themselves into more advantageous positions. We have first, then, 



