2 MORPHOLOGICAL TYPES 



The first necessity is to be clear as to what a chordate or 

 vertebrate animal is, and how it differs in plan of structure 

 from invertebrate animals (as typified, say, by Annelids or 

 Arthropods). 



A vertebrate is bilaterally symmetrical and moves typically 

 in one direction with one side constantly presented upwards. 

 A ccelom is present and the body, which is elongated from 

 front to rear, is made up of a linear series of more or less 

 similar blocks or segments. This repetition of parts or 

 metameric segmentation affects tissues derived from all three 

 of the primary layers from which the animal develops (see 

 Chapter XI). 



The gut on its way from mouth to anus is suspended in a 

 fold of ccelomic epithelium forming a dorsal mesentery. 

 The coelomic cavity can be separated into the following 

 regions. The dorsal parts of the coelomic epithelium form 

 the somites, which are segmentally arranged, and give rise 

 to plates of muscle, or myotomes, one pair to each seg- 

 ment. The portion of ccelomic space associated with each 

 myotome is a myocoel. The myoccel is bounded mesially by 

 the myotome, and laterally by the cutis-layer of the ccelomic 

 epithelium. Slightly ventral to each myotome is a region of 

 ccelomic epithelium which gives rise to excretory tubes 

 (coelomoducts). This region is the nephrotome and its cavity 

 the nephrocoel, also metamerically segmented. The ventral 

 region of the ccelom is lined by epithelium (peritoneum) which 

 forms the splanchnopleur where it is applied to the gut, and 

 the somatopleur applied to the outer wall of the body. In 

 this region the cavity is called the splanchnoccel. 



The splanchnoccel is continuous from end to end of the 

 animal, and uninterrupted by partitions or septa, except for 

 that which separates an anterior pericardial from a posterior 

 perivisceral space. This amounts to saying that the segmenta- 

 tion of the mesoderm does not persist in the ventral region. 

 In higher forms much use is made of the free and uninter- 

 rupted space afforded by the splanchnoccel for the accommo- 

 dation of longitudinal excretory and genital ducts, extensions 

 of the liver and lungs, and coilings of the gut. 



