208 



ZOOLOGY 



SECT. IV 



of Japan, at Mauritius and Madeira. Tubipora and Heliopora have 

 the same distribution as the reef -building Corals. 



From the palseontological point of view, corals are of great im- 

 portance : they are known in the fossil condition from the Silurian 

 epoch upwards, and in many formations occur in vast quantities, 

 forming what are called coral limestones. The majority of fossil 

 forms are referable to existing families, but in the Palaeozoic era 

 the dominant group was the Rugosa, the affinities of which are still 

 very obscure. In these the corallites are usually bilaterally sym- 

 metrical, the septa are arranged in multiples of four, and the cup 

 presents on one side a pit, the fossula, where the septa are greatly 

 reduced. 



CLASS IV.-CTENOPHORA. 



1. EXAMPLE OF THE CLASS Hormiphora plumosa. 

 External Characters. Hormiphora is a pear-shaped organism 

 about 5-20 mm. in diameter, and of glassy transparency (Figs. 



FIG. 159. Hormiphora plumosa. A, from the side, B, from the aboral pole, mth, 

 mouth ; s. pi swimming plates ; t. and b. tentacles. (After Chun.) 



159 and 160). The species //. plumosa is found in the Mediter- 

 ranean ; allied forms belonging either to the same genus (often 

 called Cydippe) or to the closely allied genus Pleurobrachia are 

 common pelagic forms all over the world. 



From opposite sides of the broad end depend two long tentacles 

 (t.), provided with numerous little tag-like processes, and springing 

 each from a deep cavity or sheath, into which it can be completely 

 retracted (Fig. 160, t.sh.}. At the narrow end where the stalk 

 of a pear would be inserted is a slit-like aperture, the mouth 



