CHORD ATA — VERTEBRATA 323 



The backbone in lower forms consists mainly of cartilage, in 

 the higher forms this cartilage is replaced by bone. The central 

 nervous system (spinal cord) is penetrated by a very small 

 longitudinal canal. The pharynx is perforated throughout life 

 or only in the embryo by paired branchial openings, the gill-slits. 

 The mouth is ventral and anterior, the anus ventral and pos- 

 terior. An important digestive gland, the liver, is developed as a 

 hollow outpushing of the intestine ; the blood from the intestine 

 passes through this before entering the general body circulation. 



The Vertebrata are subdivided into : — 



Division a, Acrania. — True skull absent (whence the name 

 from Greek a, negation, -{- kr anion, the skull). Brain slightly 

 developed; no heart present; blood colorless. Notochord 

 persistent throughout life, extending from end to end of body. 



This division includes only two genera, the most important 

 of which, Amphioxus, is world-wide in its distribution. No 

 fossil remains referable to this division are known. 



Division b, Craniata. — True skull present (whence the name, 

 from Greek kranion, a skull) . Brain highly developed ; a 

 pair of very complex eyes present ; heart of three or four cham- 

 bers ; blood with red corpuscles. Pharynx (with single excep- 

 tion of Bdellostoma) perforated by not more than seven pairs 

 of gill-slits ; notochord present only in the embryo, after which 

 it becomes surrounded by a segmented ring of cartilage (only 

 rods of cartilage in the Cyclostomata) ; this cartilage in higher 

 forms changes to bone. Paired limbs usually present. 



The Craniata are divided into the following classes : — 



Page 



A. Cyclostomata 337 



B. Ostracodermi ' 337 



C. Pisces 339 



D. Amphibia 349 



E. Reptilia 354 



F. Aves 369 



G. Mammalia , . . . . 373 



