Invertebrate Gallery of the Indian Museum. 37 



and are therefore seldom met with in Museum collec- 

 tions. 



In this collection they are represented at present only 

 by the alcoholic specimens already named, and by glass 

 models of Mertensia and Cestum veneris. 



Cestum veneris, however, is an unusual form, for the 

 reason that its body is much compressed and elongated. 



Cydippe 2iX\6. Beroe are the typical forms. 



METAZOA CiXLOMATA. 



We next come to the CcElomata, or animals that pos- 

 sess a true body-cavity, or coelom, through which the 

 alimentary canal runs somewhat like a lamp-chimney 

 through a lamp-globe. The coelom is a closed cavity and 

 lodges, besides the alimentary canal and the glands acces- 

 sory to digestion, at least the organs of circulation, excre- 

 tion and reproduction. 



In the Coelomata the differentiation of the cells of 

 which the body is built up has become so complicated that 

 it is only in the very earliest stages of development that 

 the simple division into three germinal layers — ectoderm, 

 endoderm and mesoderm — is to be verified. 



In a majority of the Coelomate phyla we are able to dis- 

 tinguish an anterior end or head, just behind which the 

 mouth opens ; a posterior end at which the vent opens ; a 

 ventral surface turned towards the ground upon which 

 the animal moves, and opposite to it a dorsal surface ; and 

 in most a plane carried vertically through the body, along 

 its whole length, from dorsum to venter divides the body 

 into two practically symmetrical halves. 



The first phylum, or family union, of the C(£lomata is 

 formed by the Platyhelminthes or Flat-worms in which the 

 body-cavity is imperfect and parenchymatous, and the in- 

 testine, except in the case of the one Class of Memertines, 

 either ends blindly or is entirely absent. 



