144 CCELENTERATA OR STINGING-ANIMALS. 



C. Class Ctenophora. 



Delicate free-swimming organisms, generally globular in form, 

 moving by means of eight meridional rows of ciliated plates 

 or comb-like combinations of cilia. The stinging-cells are 

 usually modified into " adhesive cells." The mouth is at 

 one pole, and leads into an ectodermic gullet. The gastric 

 cavity is usually much branched. The middle stratum is 

 well developed, and includes muscular and connective cells. 

 At the aboral pole, there is a sensory organ, including an 

 " otolith " which seems of use in steering. Here, also, there 

 are two excretory apertures. Except in Beroe and its near 

 relatives there are two retractile tentacles. All are herma- 

 phrodite. The development is direct. They are pelagic, 

 very active in habit, carnivorous in diet, and often phosphor- 

 escent. According to Lang, they have affinities with Plan- 

 arian " worms," but this view is not generally accepted. 

 Examples. — (a) With tentacles, Cydippe and the ribbon-shaped 

 Venus' Girdle (Cesium Veneris). 

 (6) Without tentacles, Beroe. 



Appendix : Mesozoa. 



We are not at present warranted in attaching much importance to 

 some very simple parasites which Van Beneden has called Mesozoa. 

 They may be very primitive and persistent modified gastruloe, hence 

 some would call them Gastrjeadie ; while Van Beneden's title suggests 

 their position between Protozoa and Metazoa. Hatschek, comparing 

 them with equal justice to precociously reproductive planulae, calls 

 them Planuloidea. On the other hand, they may have degenerated by 

 parasitism from Turbellarian worms. 



They have no mouth or alimentary cavity. The ectoderm is ciliated. 

 The endoderm consists of a single large cell (in Dicyemidse), or of a few 

 (in Orthoneclidoe), and from it the reproductive elements are produced. 

 What is known of their life-history is peculiar. 



Examples. — Dicyemidse, e.g^., Dicyema, in the kidneys of Cuttle- 

 fish. 

 OrthonectidEe, e.g., Rhopalura, in Brittle-stars, 

 Turbellarians, and Nemerteans. 



General Life of Ccelenterata. — Almost all the stinging- 

 animals live in the sea. The following are fresh-water 

 forms :— the common Hydra, the minute Microhydra with- 

 out tentacles, the strange Polypodium which in early life 

 is parasitic on sturgeons' eggs, the compound Cordylophora 

 occurring in canals and in brackish water, and the fresh-water 



