146 ACTINOZOA. 



horizontal or slightly oblique direction, proceeding 

 towards the bases of the pits in which the tentacles 

 are lodged. Before gaining these, however, each 

 * radial canal ' divides into two branches, the 

 secondary radial canals ; each of these again into 

 two others, and, thus, eight tertiary radial canals 

 are formed, which run towards the equatorial 

 region of the body, where they open at right 

 angles into an equal number of longitudinal 

 vessels, the 'ctenophoral canals,' whose course 

 coincides with that of the eight locomotive bands. 

 These canals end csecally both at their oral and 

 apical extremities. 



If, now, a comparison be made between this 

 nutrient system and that of Actinia, the digestive 

 sacs of the two organisms are clearly seen to cor- 

 respond; in form, in relative size, and mode of 

 communication with the somatic cavity. The 

 funnel and apical canals of Pleurobrachia, though 

 more distinctly marked out, are the homologues 

 of those parts of the general cavity which in 

 Actinia are central in position and underlie the 

 free end of the digestive sac. So, also, the para- 

 gastric and radial canals may be likened to those 

 lateral portions of the somatic cavity of Actinia 

 which are not included between the mesenteries. 

 Lastly, the ctenophoral canals of Pleurobrachia^ 

 the somatic chambers of Actinia appear to be truly 

 homologous, the chief difference between the two 

 forms being that while in the latter the body 

 chambers are wide and separated by very thin 

 partitions, they are in Pleurobrachia reduced to 

 the condition of tubes ; the mesenteries which 

 intervene becoming very thick and gelatinous, so 

 as to constitute, indeed, the principal bulk of the 



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