134 MUSCULAR SYSTEM. 



to bring their food towards them, and to renew the stratum 

 of water in contact with their surface, for the purpose of 

 respiration. The vibratile cilia are generally disposed around 

 the mouth of the polypus where the cilia of the tentacula 

 themselves are not vibratile, but merely lateral extensions of 

 these prehensile organs. The naked hydrae are irritable and 

 contractile in every part of the body, and their texture ap- 

 pears cellular throughout, without any distinct muscular 

 fibres. The fibrous structure is partially developed in the 

 irritable parts of some of the larger and more complicated 

 polypi, as those of the pennatula, virgularia, and lobularia y 

 and they form distinct longitudinal and sphincter bands in 

 the actinia and in the polypi of some of the larger litho- 

 phytes, as fungiae, explanaria, and car yophy Ilia, which so 

 much resemble fixed actiniae. 



Many of the acalepha swim, like animalcules, by the 

 movement of external, longitudinally-disposed, vibratile 

 cilia. These cilia are large, and generally visible by the 

 naked eye, and are distinctly supported by parallel rays, 

 like the fins of a fish ; but the muscular apparatus by which 

 they are so rapidly vibrated is not perceptible. From the 

 development of the nervous system, and even of eyes con- 

 taining a crystalline lens, seen in some of the animals of 

 this class, it is probable that the parallel fibrous striae per- 

 ceptible in the highly contractile mantle of medusae are of a 

 muscular nature. The long tentacular filaments which we 

 so frequently find developed from the periphery of the man- 

 tle possess a high degree of irritability, and are exquisitely 

 sensitive. The muscular development is less required in the 

 hydrostatic or physograde species, which are supported at 

 the surface of the sea by an air-bag, or a sac filled with a 

 gaseous secretion due to its own parietes. The large pecti- 

 nated cilia of the ciliograde acalepha have the same 

 kind of rapid vibratile motion as the minuter forms of these 

 organs seen in zoophytes and animalcules, and they continue 

 in the same manner their movements independent of the 

 rest of the animal in parts severed from the body. 



The fixed crinoid echinoderma appear to move their 

 numerous calcareous articulations by the exterior irritable 

 and secreting fleshy covering which envelopes all parts of 

 their body. In the free stellerida the nervous and the mus- 



