20 COMPARATIVE ANATOMY. 



Entozoa. Definition obscure. Water vascular system 

 present. As name expresses, its forms are parasitic. 



Annelida. Worm-like articulates. Division into rings 

 occasionally obscure or rarely absent. Limbs without articu- 

 lations. Alimentary canal floating, with anus dorsal. Cilia 

 present. Blood-vessel system complete. 



Crustacea. Aquatic articulates. Segments of head and 

 thorax combined (cephalothorax) ; intestine straight; all pass 

 through a series of metamorphoses in development all un- 

 dergo exuviation ; all respire by branchiae or by surface of 

 body performing function of aeration. (Dana.) 



Myriapoda. Air-breathing articulates. Legs numerous 

 fewer in young than adult. 



Arachnida. Air-breathing articulates. Segments of head 

 and thorax combined (cephalothorax) ; legs, eight at maturity. 



Insecta. Air-breathing articulates. Head distinct from 

 thorax ; legs, six ; body permeated with air : often winged. 



VERTEBRA (Yertebrata). 



The embryo of a vertebrate is evolved from the dorsal 

 aspect of the ovum; a chorda dorsalis (notochord) is placed 

 beneath a dorsal median groove; an internal skeleton is al- 

 ways present, composed of cartilaginous or osseous segments. 

 These segments are thought to be modified portions of a 

 number of typical vertebrae, each vertebra being composed 

 of a centrum (body), neurapophysis (lamina), neural spine 

 (spinous process), diapophysis (transverse process), zygopo- 

 physis (oblique process), pleurapophysis (rib), haemapophysis 

 (intercostal cartilage), haemal spine (segment of sternum), 

 epipophysis (spine above centrum), and hypopophysis (spine 

 below centrum). The cavity defined above the centrum is 

 the neural canal ; that below the centrum, the haemal canal. 

 By this method the plan of the Vertebrata might be compared 

 to a double ring the upper designed for the reception of the 

 neural axis; the lower for the blood-vessel and digestive sys- 

 tems, together with the sympathetic nerve. 



Motion of jaw up and down. Complete vascular and lym- 

 phatic systems with portal circulation, spleen, and valvular 

 heart present, except in Amphwxus (lancelet). The organs 



