$37. 



THE POLYPI. 



45 



tacles have only a single row of cilia, which move regularly and volunta- 

 rily, like the rotatory organs of the Rotatoria. 



By means of the currents produced by the cilia of their tentacles, many 

 Polyps draw towards their mouth light particles of food ; '•'^ others make 

 use of their ciliated arms to seize larger portions.*''' This act is aided by 

 the nettling and various prehensile organs, which are more usually found 

 upon those Polyp-arms having no cilia. *^' These organs are found upon the 

 tentacles of Actinia, Edwardsia, Veretillum and AlcyoJiium, and without 

 doubt serve for the seizing of the prey as well as its retention until death. 



But these should not be confounded with special prehensile organs found 

 on the tentacles of certain species. These consist of a small coriaceous 

 capsule, from which the animal can project a kind of sting. '^"> By means of 

 these organs, the animal can attach itself like a bur to external objects, 

 and not by suction, as is generally supposed. 



The circular or oval mouth is always situated in the centre of the an- 

 terior extremity of the body ; it is often surrounded by a lip formed of 

 circular fibres.*"' In a few species, the mouth projects like a cone at the 

 base of the tentacles.*^-' ^'\.i\ii\i.Q Plumatellae'^^^ the mouth is topped by a 

 tonguelet covered with rapidly moving ciiia. Some of the Anthozoa, 

 which capture animals of considerable size, can, in swallowing them, dilate 

 their mouth to an astonishing width.*"' 



DIGESTIVE CAVITY OF ANTHOZOA. 



§37. 



The simple stomach of Anthozoa, which is of a variable length, opens 

 in general directly external by means of the mouth,*" and with a few 

 species, only, is there a muscular oesophagus.*-' 



With some, the stomach blends with the walls of the body,*^' but usually 

 it is more or less isolated. There remains, therefore, a cavity of the body 

 of variable size, and which is directly continuous with the cavities of the 

 arms. In those Polyps living in colonies, it is prolonged into canals trav- 

 ersing the corallum, and in this way the cavities of the bodies of all the 



1 F lustra. Eschar a, Tubulipora and Crista- 

 tella. 



8 Actinina. 



9 Hydra, Coryne, Eleutkeria, Sertularia, Cam- 

 panularia and Atcyonium. 



10 Such prehensile organs have been observed by 

 ^uatrefa:;es upon the clavate tentacles of Eleu- 

 tkeria. He thinks also he has observed two mus- 

 cles in their capsules, by which the retractile sling 

 is projected (Ann. d. Sc. Nat. XVIII. 1842, p. 276 

 and 283, pi. VIII.; or Froriep's neue Notizen, 

 1843, No. 543, p. 230). The oval vesicles which 

 roughen the tentacles of Campanularia, and which 

 Lovin {IVie.gmann^s Arch. 1837, 1, p. 252) has de- 

 scribed as small spinous warts, are jirobably of the 

 same nature. In Hydra eiich hook-organ upon the 

 arm is surrounded by a group of similar vesicles, in 

 Uie interior of which is a rigid bristle. These or- 

 gans are here found only upon the arms. They 

 are distinguished from the organs having hooks by 

 their less size, and from their having no project- 

 ing filament. Corda has not properly distin- 

 guished them from the hook-organs, whose fila- 



ment is still unprojected (see his Memoir in the 

 Nov. Act. physico-medica. XVIII. p. 300, Tab. XV. 

 fig. 5, 9, 10). Perhaps the organs which Erdl 

 (Miiller's Arch. 1841, p. 424, Taf. XV. fig. 3) has 

 seen upon the tactile lobules of yeretillum cyno- 

 morium are of this kind. 



11 Actinia and Edwardsia. 



12 Hydra, Coryne. and Campanularia. 

 1'' Alcyonella and Cristatella. 



1* Actinia and Hydra. 



1 Veretillum, Alcyoninm, Actinia and Hydra. 



2 Edwardsia. See Quatrefages (Ann. d. So. Nat. 

 XVItl. pi. I. fig. 2 ; pi. II. fig. 1, 2). 



3 Hydra. The stomach of the arm-polyps is not, 

 as has been formerly supposed, a simple excavation 

 in the body. It has proi)er walls distinct from 

 those of the body, by which, however, they are 

 closely embraced. There is, therefore, in Hydra 

 no cavity of the body, and the cavities of the ten- 

 tacles open directly into the stomach. This is also 

 true of Eleutkeria (Quatre/ages, Ann. d. Sc. Nat. 

 XVUI. p. 283). 



