IV 



PHYLUM OELENTERATA 



211 



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. 158 

 and 159). The species H. plumosa is found in the Mediterranean ; 

 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 

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



FIG. 158. Hormiphora plumosa. A, from the side, B, from the aboral pole. mth. mouth ; 

 s. pi. swimming plates ; t. and 6. tentacles. (After Chun.) 



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

 retracted (Fig. 159, t.sh.}. At the narrow end where the stalk 

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

 (mth,) : this end is therefore oral. At the opposite or aboral pole 

 is a slight depression, in which lies a prominent sense-organ (s.o.), 

 to be described hereafter. 



But the most striking and characteristic feature in the external 

 structure of Hormiphora is the presence of eight equidistant meri- 

 dional bands (s.pl.), starting from near the aboral pole, and extend- 

 ing about two-thirds of the distance towards the oral pole. Each 

 band is constituted by a row of transversely arranged comb-like 

 structures, consisting of narrow plates frayed at their outer ends. 

 During life the frayed ends are in constant movement, lashing to 

 and fro, and so propelling the animal through the water. The combs 



P 2 



