142 ACTINOZOA. 



within which the nutritive products circulate. In 

 MeandHna and certain other Zoantharia, the 

 general cavities of the polypes open by wide aper- 

 tures into each other ; but in very many forms of 

 coralligenous Actinozoa it is erroneous to suppose 

 that any connection, available for nutrient pur- 

 poses, is maintained between the different polypes 

 of the same compound stock. 



Although, in a large number of Actinozoa, the 

 somatic cavity, has no communication with the 

 exterior, save through the digestive sac or the 

 free ends of the tentacles, when these are perforate, 

 yet, among other members of the class, the exist- 

 ence of apertures in the body-wall seems to have 

 been satisfactorily ascertained. Mention has al- 

 ready been made of the cinclides of Actinia, nor 

 are these orifices wanting in several allied genera. 

 In Philomedusa, according to Fritz Miiller, twelve 

 rows of such openings, which appear to the naked 

 eye as minute pale dots, and capable of inde- 

 pendent contraction, radiate from the posterior 

 extremity of the animal. In the centre of this 

 extremity, as also in Peachia and Cerianthus, 

 exists a much larger aperture, or, rather, short 

 canal, which the animal has the power of closing 

 effectually whilst its somatic cavity remains dis- 

 tended with fluid. And Milne Edwards, has 

 shown that in Corallium the coenosarcal canals 

 communicate directly with the surrounding me- 

 dium by means of numerous perforations in their 

 walls. Lastly, to the category of structures now 

 under consideration must be referred the ' apical 

 pores ' of the Ctenophora, whose nutrient system 

 presents peculiar featm^es which render it necessary 

 that some account of Pleurobrachia ( = Cydippe), 



