POLYPS. 71 



Of the nervous system of Polyps little is known ; it has not 

 been described as a connected whole, but different observers have 

 imagined that they had discovered ganglia or a nervous ring near 

 the mouth. From analogy it is probable that the nervous system, 

 where it exists, does form a ring round the mouth ; and that the 

 threads, which SPIX described in the pedal disc of Actinice as 

 nerves, can on this account scarcely be considered to be such. As 

 organs of sense, four coloured spots at the edge of the disc have 

 been observed in the Medusa-form products of Syncoryne and Coryne 

 fritillaria of STEENSTRUP, which entirely resemble the parts that 

 EHRENBERG considers to be eyes in Medusa. Still more distinct is 

 this organ in a form described by QUATREFAGES, which he names 

 Eleutheria dichotoma. Here this author found six eyes with a 

 hemispherical lens, a granular pigment of a red colour, and a 

 spherical projection of the integument closing the eye like a comea 1 . 



In Bryozoa the muscular system is most largely developed, and 

 serves principally to retract the animal within its cell. It protrudes 

 itself partly by straightening the alimentary canal, partly by means 

 of transverse muscular fibres which contract the diameter of the 

 visceral cavity, and in that way elongate it 2 . Traces of a muscular 

 system have also been met with in other Polyps. 



As to the geographic distribution of Polyps, we have only 

 imperfect notices. Fresh-water Polyps, Hydra, Alcyonella, Plu- 

 matella, have, as far as I know, only been observed in Europe ; but 

 from this to conclude that they do not occur in tropical regions, 

 would probably be premature. Still EHRENBERG found no Hydros 

 in Africa and Arabia. Actinice are met with in all seas. The 

 Polyps with polyparies, which are almost all inhabitants of the sea, 

 are richest in species in warm regions ; Flustra, amongst the 

 Bryozoa, is it seems an exception to this : the European species of 

 this genus are about as numerous as the foreign : these last come 

 principally from New Holland; Gorgonia occurs in all seas, but 



VAN BENEDEN, Mem. sur les Campanulaires de la cdte d'Ostende, M6m. de TAcad. 

 Royale de Bruxelles, xvn. 1843. Ann. des Sc. natur., sec. Serie. Tom. xx. Zool. pp. 



350373- 



DUJARDIN, Memoire sur le developpemcnt des Meduses et des Polypes hydraires. Ann. 

 des. Sc. Nal. $e Serie. Tom. iv. 1845. Zooloyie, pp. -257 281. PI. xiv. xv. 



1 Ann. des Sc. Natur. sec. Serie. Tome xvm. p. 280. PI. vm. fig. 6. 



2 Minutely detailed by FABRE in JBowerlankia, Phil. Transact-. 1837. pp. 393396. 



