212 Mr. W. S. Kent on the Relationship of 



type of structure which obtains tliroughout the Actinozoa ; we 

 now turn to the sponge tribe to ascertain what its represen- 

 tative shares in common with it. 



Fig. 2 is supposed to represent a transverse section of a 

 liiglily developed sponge with a single excurrent aperture, 

 as at fig. 1, taken through a similar region as the section at 



fig- 4. 



A glance is sutlicient to show us at once that we have here 



something entirely irreconcilable Avith what obtains in the 

 corresponding section of an Actinozoon, and very few words 

 will suffice to indicate how sharply defined and " thorough- 

 going " arc the points of distinction. 



The most striking of all the ])licnomena presented are, per- 

 haps, the perforations in the body- wall, /',y,y (assuming for 

 tlie nonce that this sponge-body is a distinct individual) ; these 

 apertures are as essential to the existence of the s])onge as the 

 single terminal buccal orifice is to i\\Q, Actinia^ and are, in fact, as 

 has been already shown, the channels through whieli it derives 

 all matters of nutrition. Next we have the interstitial system 

 of canals pervading the whole body -mass, intercommunicating 

 with each other in every direction, and finally debouching in 

 the common excurrent cavity h. Can we be said to have here 

 any thing homologous or even analogous to the double tube 

 and symmetrical mesenteric system of the Actinozoon ? Ilackel 

 endeavours to surmomit the difficulty of this peculiar and es- 

 sential incurrent porous system of the sponge by supposing 

 the cuticular pores in connexion with the somatic cavity oc- 

 casionally met with in some Actinice to be its homologue ; but 

 these cuticular pores of the sea-anemone are exceptional, and 

 by no means an essential portion of the animal's stmcturc, 

 and much less are they subservient to its functions of nutrition. 

 He presumes, again, that these cuticular pores may be constant, 

 though hitherto unobserved, in all the Actinozoa, and that 

 cuiTcnts of water, serving respirator}' purposes, are constantly 

 passing through them into the general stomachal cavity ; and 

 taking this for granted, he, in the next paragraph, asserts, 

 as a matter of positive fact, " that an essential morphological 

 difference does not exist between the nutrient vascular system 

 of the sponges and corals ;" that both (solitary individuals) 

 possess a central cavity or stomach, which opens outwards by 

 a single large orifice (the osculum or mouth), from which 

 cavity canals issue in all directions, which traverse the body- 

 wall, and finally open on the surface by the cutaneous pores. 



This assertion is built up on a framework of mere hypo- 

 thesis ; and its entire fallacy is proved by the fact that the 

 largest section of the coral-formmg Actinozoa, the whole of 



