POLYPI. 85 



and the absence of any hitherto recognised nervous filaments, mani- 

 fests an obvious predilection for light, and, when confined to a glass, 

 always moves itself to the brightest side. Trembley succeeded in in- 

 verting one of these delicate animalcules, and the creature soon ac* 

 commodated itself to this singular change in its condition : digestion 

 being effected as actively by the surface which before was external, 

 as by that which had been the digestive surface ; whilst this as readily 

 assumed the ordinary gemmiparous function of the skin. 



The Hydrae are not less remarkable for their power of gene- 

 ration than for that of regenerating mutilated parts. They have 

 been observed to multiply by spontaneous fission, dividing them- 

 selves transversely : but the most ordinary process of generation 

 is by the development of young polypi, like buds, from the ex- 

 ternal surface of the old one. It is, however, most probable that 

 in these cases the gemmation is preceded by the development 

 and fecundation of the true ovum, beneath the integument. The 

 Hydra unquestionably presents a periodical development of sexual 

 organs of two kinds : one, at the anterior or oral extremity of 

 the body consists of small nodules or sacs, which Ehrenberg dis- 

 covered to contain moving filaments, or seminal animalcules : another 

 series of cells, developed in the posterior part of the stem, contain 

 ova, which, after impregnation, are discharged, but sometimes are 

 retained, and then grow out like buds. Sometimes one individual 

 Hydra developes only the male cysts, or sperm-vesicles ; sometimes 

 only the female ones, or ovisacs ; but the rule is generally to have 

 both kinds. 



The seas which wash our own shores are tenanted by numerous 

 forms of minute Polypi, having essentially the same simple organis- 

 ation as the Hydra ; but which are protected from the dense briny 

 element surrounding them by an external horny integument (^Sertu- 

 larians). Now these likewise develope new polypes by gemmation ; but, 

 as the external crust grows with the growth of the soft digestive sac, the 

 young polype adheres to the body of the parent, and, by successive 

 gemmations, a compound animal is produced. Yet the pattern 

 according to which the new polypes and branches of polypes are 

 developed is fixed and determinate in each species ; and there conse- 

 quently results a particular form of the whole compound animal or 

 individual by which the species can be readily recognised. This 

 hydriform polype-animal, or association of polypes, resembles a 

 miniature tree ; but consists essentially of a ramified tube of irritable 

 animal matter, defended by an external, flexible, and frequently 

 jointed, horny skeleton ; fed by the activity of the tentacula and by 

 the digestive powers of the alimentary sacs of a hundred polypi, the 



G 3 



