CHAPTER XV 



CTENOPHORA 



The Ctenophora are spherical, lobed, thimble-shaped, or band- 

 like animals, usually very transparent and gelatinous in structure. 

 They are exclusively marine, and are found floating at or near 

 the surface of the sea. 



Although they are generally classified with the Coelenterata, 

 they are regarded by some authors as having closer affinities 

 with the Polyclad Turbellaria (cf. Vol. II. p. 7). They agree, 

 however, with neither of these divisions in their essential 

 characters, and the only way to indicate and emphasise their 

 unique position is to place them in a separate Phylum. 



They differ from all the Coelenterata in the absence of 

 nematocysts, and in the presence in development of a definite 

 mesoblast. The character from which they derive their name, 

 Ctenophora, is the presence on the surface of bands of swimming 

 plates. The plates are called the " combs " (/cret?, gen. /crez'o? = 

 a comb) or " ctenophoral plates." They occur in all genera included 

 in the Phylum except in Coeloplana (Fig. 183, p. 422). 



Another peculiarity of all Ctenophora (except the Beroidae) 

 is the presence, at some stage in the life-history, of two long 

 and extremely contractile tentacles. There is also a well- 

 developed sense-organ (statocyst) in the centre of the aboral 

 area of the body. 



The Ctenophora differ from the Turbellaria in the presence 

 of the combs and of the two long tentacles, in the position and 

 relative importance of the statocyst, and, with the exception of 

 Coeloplana, in the general characters of the alimentary canal. 



Shape. — Several of the Ctenophora are conical or spherical 

 in shape, but exhibit at the pole where the mouth is situated 



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