

CHAPTER IX 



INTRODUCTION TO THE COELOMATA 



IN the groups of Metazoan animals so far studied we have to 

 deal with an external layer of cells called the ectoderm (or in 

 Porifera the dermal layer) which forms the skin, and an internal 

 layer termed the endoderm (or in Porifera the gastral layer), whilst 

 between them is found a space containing a substance which varies 

 from the condition of a semi-solid jelly in Coelenterata to that of a 

 fluid which is almost pure water in Rotifera. In addition except in 

 Coelenterata it contains cells which are developed into processes 

 which stretch across the cavity. And these processes may secrete 

 fibres and become connective tissue or may become developed into 

 muscular fibres. Where the cells and fibres are sparse this space 

 is said to be a primary body-cavity. Where they are abundant 

 it is called parenchyma or connective tissue. 



The phyla which are next to be considered, and which may be 

 grouped together under the name Coelomata, differ 

 from those which we have so far considered in the 

 possession of an important organ termed the coelom (Gr. KoiA.a>/m, 

 a thing hollowed out). This, like the primary body-cavity, is often 

 described as a space intervening between the ectoderm and endo- 

 derm, and the terms coelomic cavity and body-cavity have been 

 used to describe it. In spite of the etymological difficulty we 

 propose in the following pages to deal with this organ under the 

 term coelom, and its cavity under the term coelomic cavity. In 

 reality it consists of one or more pairs of sacs with perfectly defined 

 walls lying at the sides of the endodermic tube. In the adult these 

 sacs join each other above and below the endoderm, and the adja- 

 cent walls entirely or partly break down, and thus one continuous 

 cavity results. The wall of the coelom and the tissues derived from 

 it are known as the mesoderm. To describe the coelom as a split 



