164 



ACTINOZOA. 



in this organism strongly suggests the possibility 

 of its being an immature form, nor is the suspicion 

 weakened by the discovery of Haime, that the 

 young of Cerianthus, while resembling Arach- 

 nactis in physiognomy, enjoys a similar oceanic 

 mode of life. 



The muscular fibres of the Actinozoa are in- 

 teresting to the histologist, as wanting, among 

 many forms, those distinct transverse striae, which 

 elsewhere, they so frequently present. Such stride 

 are not, however, always absent. In the body- 

 substance of this class we have, in truth, obvious 

 transitions from a simple contractile periplast to 

 muscular fibres, which in no essential respect 

 differ from those of various invertebrate animals. 

 In the typical Ctenophora, the contractile tissues 

 appear to be disposed in two principal sets; a 

 transverse or circular, and a longitudinal. 



Some Zoantharia employ their tentacles as 

 aids to locomotion, though neither in these nor in 

 the Alcyonaria can it rightly be said that special 

 motile organs exist. 



Of this nature, however, are the < etenophores,' 

 or ciliated bands, which constitute so obvious a 

 feature in the physiognomy of the Gtenophora. 

 The normal number of these bands would seem to 

 be eight, though in Cestum, and one or two other 

 forms, their typical structure and arrangement is 

 somewhat modified. Each ctenophore is of a 

 much elongated ovate form, widest at the equa- 

 torial region of the body, and tapering gradually 

 to end in a point at some distance from the oral 

 and apical poles ; slight differences in degree of 

 approximation to these parts, and such-like minor 

 characters, distinguishiug the ctenophores of the 



