198 



ZOOLOGY 



SECT. TV 



CLASS IV. CTENOPHORA. 



1. TYPE OF THE CLASS Hormiphora plumosa. 



External Characters. Hormiphora is a pear-shaped organism, 

 about 5-20 mm. in diameter, and of glassy transparency (Figs. 145 

 and 146). 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 

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

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



FIG. 145. Hormiphora plumosa. A, from the side, B, from the aboral pole. mth. mouth; 

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



retracted (Fig. 146, 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, m 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 extending 

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

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

 tures, consisting of narrow plates frayed at their outer ends. During 

 life the frayed ends are in constant movement, lashing to and fro r 



