IV 



PHYLUM ( '(ELF.NTF.i: ATA 



227 



such as the presence in the \\all of the Sponge, of the system of 

 inhalant apertures, the presence of the peculiar collared endoderm 

 cells, and the absence of stinging-capsules, would suffice to remove 

 the Sponges from the Ccelenterata, and place them in a phylum 

 apart. But not only is the grouping of Sponges and Coelenterates 

 in one phylum thus rendered impossible by important differences 

 in their structure and development ; a comparison of the mode 

 of formation of the embryonic layers in the two groups shows such 



Cestida 

 Lobata 



Hexactinia Madreporaria 



Cydippida 

 Platyctenea 



Rhizostomeue Semostomifc 

 Beroida 



Edwards; a 



Coronatct 



onaris 



Cubomedusce 



ACTINULA 



Lucernarida 



Hydrocorallin* 



Leptolince 



SCYPHULA 



HYDRULA 

 Fio. 177. Diagram illustrating tin- mutual relationships of the Co-leuterata. 



radical dissimilarity that it is scarcely possible to find sufficient 

 evidence for regarding them as having been derived from the same 

 metazoan ancestors, and there is much to be said in favour of the 

 view that they have originated separately from the Protozoa. 



APPENDIX TO THE CCELENTERATA. 



THE MESOZOA. 



Under the designation MESOZOA have been comprised certain lowly organ- 

 ised animal forms, formerly supposed to afford us something of the nature 

 of a connecting link between the Protozoa and the Metazoa, but now more 

 generally looked upon as degenerate members of the latter subdivision. It has 

 been proposed to term them the Momloidea, from the resemblance which they 

 bear to the morula stage in embryonic development. 



They are all multicellular, with an " ectoderm " composed of a single layer 

 of cells ciliated in whole or in part, and an " endoderm " either composed of a 

 single elongated cell or of several cells : a mesogloea is not represented. The 

 Mesozoa comprise at least three families, the Dicyemidce, the Heterocyemidce, 

 and the Orthonectidce, all the members of which are internal parasites. 



The Dicyemidce are parasites in the kidneys of various Cuttle-fishes and 

 Octopods (Cephalopoda). Dicyema (Fig. 178), the length of which is between 

 0-75 and 6 or 7 millimetres, consists of a head -part or calotte, and an elongated 



Q 2 



