230 



ZOOLOGY 



APPENDIX (II.) TO THE CCELENTERATA. 



The Mesozoa. 



Under the designation Mesozoa have been comprised certain lowly organised 

 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 Moruloidea, 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 mesoglcea is not represented. The Mesozoa 

 comprise at least three families, the Dicyemidce, the Heterocyemida', and the 

 Orthonectidce, all the members of which are internal parasites. 



The DicyemidcK are parasities in the kidneys of various Cuttle-fishes and 

 Octopods (Cephalopoda). Dicyema (Fig. 175), the length of which is between 



Fir.. 1V5.— Dicyema paradoxum, 



with infusuiiform embryos (males). 

 (Frum Bronu's Thicvi-clch, aftei' 

 Kulliker.) 



Fig, iVii.— Dicyema paradoxum, 



with vermiform embryos. (From 

 Brunn's Tkicrrdch, after KoUiker.) 



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

 body. The form of the calotte varies a good deal, according to age ; in young 

 specimens it is isotropic (j.e. symmetrical around the long axis) ; in the adult 



