MULTICELLULAR ANIMALS 



147 



The sub-phylum Vertebrata constitutes a well-defined group with 

 many similarities. They are in general so distinctly different from 

 animals that lack a vertebral column that it is convenient to make 

 two divisions of animals and to distinguish between the inverte- 

 brates and \^RTEBRATEs. The invertebrates include all animals of all 

 phyla, including three chordate sub-phyla and excepting only the 

 members of the sub-phylum Vertebrata. 



In the more primitive types of vertebrates, for example, the shark, 

 the skeleton is made up of cartilage, a tissue that has a great deal of 



NEURAL TUBE 

 NOTO CHORD 



ECTODERM 

 ENDODERM 



SEGMENTATION 

 CAVITY 



SOMATIC 

 MESODERM 



-COELOM 



ARCHENTERON 

 {PRIMITIVE GUT) 



SPLANCHNIC 

 MESODERM 



Fig. 95. — Diagrammatic section of a portion of a vertebrate embryo to show espe- 

 cially the position of the notochord. 



rigidity but is also elastic (Fig. 96). In the true fishes many of the 

 cartilaginous elements are replaced by bone (Fig. 97), by the re- 

 placement of the cartilage cells by bone cells, and the deposition of 

 lime salts in the intercellular spaces. In the fishes also, some of the 

 dermal bones of the exoskeleton become involved with the endo- 

 skeleton, particularly in the region of the head and jaws. The endo- 

 skeleton of the higher vertebrates is thus composed of bones of two 

 different origins, cartilage bones, which are the result of the re- 

 placement of the primitive cartilages by bone, and dermal or mem- 

 brane BONES, which are derived from the exoskeleton and do not 

 pass through a cartilage stage during development. 



