Group 8. Vertebrata. 



Greneral review. The Vertebrata are bilaterally symmetrical. 

 There is a dorsal central nervous system, of which the 

 anterior end is usually enlarged to form the brain, whilst the- rest 

 forms an elongate spinal cord. Below the nervous system lies 

 another cord, the notoohord or chorda dorsalis ; skeletal struc- 

 tures are usually developed round these two. Below the notochord is 

 the alimentary canal, the mouth is anterior, the anus ventral, usually 

 some little distance from the posterior end. The heart lies anteriorly 

 below the digestive tract. There is a pair of kidneys and a pair 

 of gonads; the ducts of both these open near the anus, or into the 

 posterior portion of the gut. Alimentarj canal, heart, etc., lie 

 in a spacious body-cavity. Optic, auditory and olfactory 

 organs, are present anteriorly. The body is naturally divisible 

 into the following regions: (1) the head, with the brain, sense- 

 organs, and mouth; (2) the trunk, extending from the head to 

 the anus, and enclosing the body-cavity with its contained organs, and 

 usually furnished with two pairs of appendages, the limbs ; these last, 

 especially in the higher Vertebrata, play an important part in 

 locomotion; (3) the tail, the muscular termination of the body, 

 forming a powerful locomotor organ in the Pisces, but usually of 

 subordinate importance in the higher Vertebrata. In the higher forms, 

 from Ileptilia upwards, the anterior portion of the trunk forms a n e c k; 

 that is, the body-cavity is drawn away from the anterior end of the 

 trunk ; and organs {e.g., the heart) which in other forms occur there, 

 have also moved back, so that this portion is practically without viscera, 

 and forms a muscular stalk -like connection between head and trunk, 

 which is of the greatest importance in connection with the free 

 movement of the head. 



The epidermis consists, in Amphioxus, of a simple epithelium 

 of cylindrical cells ; in other Vertebrata there is a stratified squamous 

 epithelium of varying thickness. In Pisces all the epidermal cells 



