INNER STRUCTURE OF PRIMITIVE VERTEBRATES. 26 1 



separated into two distinct cavities, which are completely 

 divided by a transverse partition, the muscular midriff, or 

 diaphragm. The first, or chest-cavity, contains the oesopha- 

 gus, the heart, and the lungs ; the other, the ventral cavity, 

 contains the stomach, small intestine, large intestine, liver, 

 spleen, kidneys, etc. But in mammalian embryos, these 

 two form a single connected cavity, a simple coelom, before 

 the diaphragm is developed, and this we find to be the 

 case in all lower Vertebrates throughout life. This coelom is 

 covered by a delicate layer of cells, the intestinal epithelium. 

 The most important of the viscera within the body- 

 cavity (coeloma), is the nutritive intestinal tube, the organ 

 which forms the whole body of the Gastrula. This is a 

 long tube, more or less differentiated, enclosed in the coelom, 

 and having two openings; a mouth-opening for taking in 

 food (Fig. 59, 60, md), and an anal opening for discharg- 

 ing waste-matter or excrement (a/). Numerous glands, all 

 of which proceed from the intestine, are attached to the 

 intestinal canal, which are of great importance in the verte- 

 brate body. These are the salivary glands, lungs, liver, 

 and numerous smaller glands. A pair of simple liver- 

 pouches (Fig. 59, 60, I) were probably present even in the 

 parent-form of Vertebrates. The walls of the intestinal 

 canal and of all these appendages, consist of two very 

 different parts or layers ; the inner cellular covering is the 

 intestinal-glandular layer, or the fourth germ-layer ; the 

 outer fibrous envelope, on the other hand, proceeds from 

 the third germ-layer, the intestinal-fibrous layer ; it is 

 mainly composed of muscle-fibres, which effect the digestive 

 movements of the intestine, and of a tissue of connective 

 fibres forming a firm covering. The mesentery, a thin, 



