196 



(schizocoele) in the sheet around the gut is the splanchno- 

 coele, which becomes the coelome or body cavity (schizo- 

 coelic and secondary in origin). Between the cavities of 

 the dorsal myotome segments and the splanchnocoele, 

 there remain for some time narrow connections, the nephro- 

 tornes, from which the nephridia or kidney tubules are 

 formed. The nephro tomes correspond with the gonad 

 sacs of Amphioxus (see paye 189). 



In the adult the coelome or body. cavity forms the peri- 

 toneal cavity of the body ; it contains the chief viscera, 

 and its lining (peritoneum) consists of inner coelomic 

 epithelium and outer connective tissue. Portions of this 

 cavity become separated off ; one of these is the anterior 

 pericardium which surrounds the heart. In lower Craniates 

 (e.g., Dogfish) communication between the pericardium 

 and the main peritoneal cavity is perpetuated through 

 a backward extension of the pericardium, the pericardio- 

 peritoneal canal, which opens into the main peritoneal 

 cavity on the ventral surface of the oesophagus. 



In Amniota portions of the cavity are shut off to form 

 the pleural cavities around the lungs. 



What is the Nature and Origin of the Skeleton of Vertebrates ? 

 Mention its principal Parts. 



It is mainly internal, an endoskeleton. Certain parts 

 (e.g., scales, feathers, hairs, teeth) are external and con- 

 stitute an exoskeleton, which is, hoAvever, not a cuticle 

 formation as in Invertebrates. (For origin of scales, etc., 

 see Integument or Skin, page 194.) In the vertebrate 

 embryo, and in adult lower vertebrates (e.g., Cyclostomata 

 and certain Fishes) the endoskeleton is cartilaginous ; 

 in adult higher vertebrates the elastic cartilage is supple- 

 mented or is largely replaced by rigid bone. 



The foundation of the skeleton, the notochord, a rod- 

 like series of turgid cells with a double membranous sheath, 

 is developed from the endoderm. The notochord extends 

 from the tip of the tail forwards as far as the infundibulum 

 of the brain. The vertebral column (replacing the noto- 

 chord) and additional skeletal parts, which together form 

 the supporting framework of the body, are of later origin, 

 being produced by secondary development from connective 

 tissue or mesenchyme (see. Amphioxus, page 192,) which 

 is cartilage-forming (i.e., the cells secrete an intercellular 

 matrix of a firm elastic substance, chondrin). But the 

 matrix may become rigid through infiltration with lime, 

 or the tissue may be invaded or extensively replaced by 



