igo AN INTRODUCTION TO ZOOLOGY 



in which is a rich capillary network. They are embedded in a matrix 

 of connective tissue, which is pigmented on the dorsal side. The 

 muscle fibres themselves consist of long narrow-pointed cells, each 

 with a nucleus situated in a central core of unmodified protoplasm 

 surrounded by a layer of finely striated contractile material. 

 4. These are followed by a much thicker layer of longitudinal muscle 

 fibres. They are also embedded in connective tissue and arranged 

 in a very characteristic manner on both sides of fine radially disposed 

 partitions. 5. Lastly, we find a thin layer of very flattened cells 

 forming the somatic part of the coelomic or peritoneal epithelium. 

 Within this complex body wall is a large space, the body cavity or 

 ccelom, in which lie the gut, the blood-vessels, the excretory organs, 

 etc., and which is not represented in any way in Hydra. 



The wall of the gut is also complex, being composed of four layers. 

 i. Lining the inside of the alimentary canal is a single layer of long 

 columnar epithelial cells, most of which are ciliated, but quite a 

 fair number are glandular. They are over the greater part of its 

 length entodermal, and constitute the enteric epithelium. 2. Out- 

 side this is a thin layer of circular muscle fibres, by means of which 

 the peristaltic movements of the alimentary canal are produced. 

 They are also supplied on their inner side by the rich sub-epithelial 

 capillary plexus, which plays a large part in the activities concerned 

 with digestion. 3. A very thin layer of longitudinal muscle fibres 

 surround the circular ones. 4. Lastly, these in their turn are covered 

 by the visceral or splanchnic part of the peritoneal epithelium. 

 In the intestinal region the cells of this layer are transformed into 

 the striking chlorogogen cells, whose appearance and structure has 

 already been dealt with. 



Thus in Lumbricus between the ectoderm and entoderm 

 there is a well-developed third cellular layer, the mesoderm, com- 

 plexly differentiated into muscles and epithelium, and divided into 

 a somatic portion, forming the main part of the body wall, and a 

 splanchnic part, helping to build up the gut wall, and between these 

 two is the large ccelom. This cavity exhibits the three main 

 characteristics of any true ccelom ; it is lined by mesoderm and 

 surrounds the gut, the gonads are produced by its epithelial lining, 

 and the primitive excretory tubules open into it. Such a type of 

 structure is quite characteristic of all the higher Ccelomata, although 

 in some groups it may become secondarily modified. 



Further differences are to be noted between Hydra and Lumbricus. 

 The latter form has a well- developed muscular system composed of 

 muscle fibres and not of epithelio-muscular cells. The alimentary 



