IIB Coelentera. 



adapted to this peculiar mode of life, so that it is possible to speak 

 of a special reef fauna. Coral reefs are a very characteristic feature 

 of tropical seas. 



1. Almost the only representatives of the order occurring in northern seas, 

 are the Actvniaria (Sea-anemones, Sea-roses) ; sohtary forms without skeletons, 

 mostly of relatively large size, and usually with several whorls of tentacles. 

 They have a broad pedal disc below, by which they attach themselves to foreign 

 objects ; they are capable of slow locomotion. Several species on English coasts. 



2. The Madreporaria, foi-ms provided with a calcareous skeleton, belong 

 almost exclusively to warm seas, where they occui- in great abundance, usually 

 in colonies, but occasionally sohtary ; a few species are met with in the 

 MediteiTanean. 



Class 3. Ctenophora. 



The Ctenophora may be regarded as medusae from which the manubrium is 

 absent, and the umbrella so convex and so much narrowed that the sub-umbrella 

 cavity forms a tube of varying width, at the base of which lies the mouth, the 

 entrance into the ahmentary canal. Eight narrow streaks may be seen passing 

 round the surface of the body like the meridians of a globe ; each of these 

 lines or ribs, as they are called, is composed of a series of tiny laminae, con- 

 sisting of transverse rows of cUia fused together ; these plates constitute the chief 

 locomotor apparatus of the animal. Many of the Ctenophora possess two long 

 branched tentacles, which arise from opposite sides of the body, and can 

 be withdrawn into special pouches. There are no other appendages. The mouth 

 leads into a small digestive cavity, the so-called infundibulum, from which 

 arise other canals running along the ribs. At the upper pole of the body there 

 is an auditory organ. Thread cells are absent. 



An eight-rayed plan is to a certain extent indicated, but is not followed 

 throughout. As a matter of fact, the body can only be divided into two 

 symmetrical portions : a, two-rayed arrangement is seen, e.g. in the disposition 

 of the tentacles, the branches of the ahmentary canal, etc. 



The Ctenophora, of whose structure only certain important points have been 

 noticed above, whilst others have been altogether ignored, are hermaphrodite 

 without an alternation of generations, and occupy a somewhat isolated position 

 among the Coelentera. They mostly occur in warm seas ; all are pelagic. 



It is specially interesting to note that the larvae of some forms often attain 

 sexual maturity quite soon after leaving the egg, and may lay f ei-tile ova ; but 

 this- only occurs during the wai-m seasons : the sexual organs then degene- 

 rate, and the larvae develop into normal individuals, which again become 

 sexually mature (c/. psedogenesis of Insects). 



Of the various forms may be mentioned Bero'e, ban-el-shaped, with a wide sub- 

 umbrella cavity, without tentacles ; Cydippe, spherical, with a narrow cavity and 

 long tentacles; Cestus veneris (Venus's girdle), with body much compressed 

 and elongated to a hgamentous form structure. All these occm- in the 

 MediteiTanean, the fii-st two also on English coasts. 



Appendix to the Coelentera: 



SpOngiSB or Porifera {Sponges). 



The Sponges form a very peculiar group of animals of low organisa- 

 tion, which is usually placed near to the Coelentera, but whose 

 systematic position is not yet determined. 



