Phylum VIII. VERTEBRATA 



Bilaterally symmetrical animals, with a cartilaginous or ossified vertebral axis, 

 which is usually composed of a series of similar segments, the same supporting the 

 central nervous system and dividing the trunk into a dorsal and a ventral portion. 

 Never more than two pairs of limbs. 



The vertebral column develops from a rod-like cellular tissue of gelatinous 

 consistency (chorda dorsalis), of which the outer (skeletogenous) layer gradually 

 subdivides into a number of similar segments, which are originally cartilagin- 

 ous but subsequently calcified by the deposition of phosphate of lime or are 

 replaced by bone substance. At the anterior end of the vertebral column is 

 the cranial capsule enclosing the brain, also the visceral skeleton. The 

 appendicular skeleton is likewise preformed in cartilage, and among the higher 

 Vertebrata usually develops completely into bone substance. Only some of 

 the lowest Vertebrata exhibit a persistently cartilaginous internal skeleton. 

 The calcification of the cartilage in the sharks and skates takes place in such 

 a way that a homogeneous limey substance is deposited between the cartilage 

 cells, while in ossification the original cartilage cells disappear, and by re- 

 sorption of the intercellular substance there arise canals with blood vessels 

 (Haversian canals) as well as small hollow spaces (lacunae, bone corpuscles) 

 filled with bone cells (osteoblasts). The latter are connected with the 

 Haversian canals by very fine tubules (primitivrbhrchen) radiating in all 

 directions. In many fishes the lacunae (bone corpuscles) are wanting, so that 

 the primitive tubules arise directly from the Haversian canals. 



The skeleton of the extremities consists of several articulated segments 

 which exhibit an extraordinarily varied arrangement according to the function 

 of the limbs. 



The nervous system consists of a central organ divided into brain and 

 spinal chord, from which numerous nerves arise and extend throughout the 

 whole of the body. The blood is first driven to the organs of respiration 

 (gills or lungs) by means of a heart provided with one or two auricles, and 

 after it has traversed the body in numerous veins it returns to the heart. 

 The oesophagus, stomach, intestine, liver, kidneys, and spleen, as well as the 

 organs of generation, lie in the ventral part of the body. The skin is often 

 provided with hairs, spines, scales, feathers, or bony plates. 



As a rule only remains of the bony skeleton, teeth, or hard ossified parts 

 of the skin are available to the palaeontologist for investigation ; but these 

 can generally be determined with great certainty. 



The following five classes of Vertebrates are distinguished : — Pisces, 



VOL. II B 



