MORPHOLOGY OF HIGHER TYPES 



139 



bones is the acetabulum. In it is seated the head of the femur (thigh 

 bone). 



The bones of the arm and leg or fore and hind hmbs are arranged 

 according to the same plan and may be compared bone for bone, humerus 

 with femur, radius and ulna with tibia and fibula respectively, carpal 

 (wrist) bones with tarsal (ankle) bones, metacarpals with metatarsals 

 (body of hand and foot respectively) and phalanges 

 (bones of the digits) of hand with those of the foot. 

 Vertebrates with primitive limbs have five digits on 

 fore and hind feet, but the limbs of specialized animals 

 have undergone more or less extensive modifications 

 from the original five-fingered and five-toed plan. 



Nervous System. — The function of the nervous 

 system is to receive stimuli and transmit impulses. 

 Not present in the Protozoa and Porifera, nervous 

 tissue is first found in the Ccelenterata. In the 

 hydroid polyps there is little specialized nervous 

 tissue, but many ectodermal cells (Fig. 108) have 

 elongated processes at their bases which serve the 

 dual function of contraction and transmission of 

 stimuli and are therefore properly called neuromuscu- 

 lar cells. Connected with these cells are sometimes 

 other cells with long processes which serve as nerves 

 for transmission of impulses. 



Degrees of Centralization in Nervous Systems. — 

 In jellyfishes, a ring of nervous tissues encircles the 

 rim of the umbrella. ^ Nerve cells of the ring are con- 

 nected with a network of nerve cells in the umbrella, ^^^- 109.— Nervous 



mi • , c system of a roundworm, 



makmg a dinuse nervous system. Ihis type of diagrammaticaily rep- 

 nervous system is the lowest in the animal scale, ''^'^^"♦''d- .4, anus; 



, • f 1  ,, ^0. anal ganglion; 5n, 



A more centralized nervous system is found in the ventral nerve; c, lateral 

 flatworms and roundworms. In the flatworms, for ef^gi^on of nerve ring; 



1 • • 1 1 m 1 11 • ""' dorsal nerve; S, 



example m the rhabdocoele and triclad rurbellaria, anterior lateral nerv^e; 

 there is a single (7an<7?to?i or thickened mass of nervous ^' sub-lateral nerve; 



, . 1 p 1 T-i 1 • '^'"' sub-median nerv-e. 



tissue in the anterior end of the worm. From this (From Sedgwick's stu- 

 ganglion (Figs. 81 and 82) two main nerve trunks dent's Textbook of Zooi- 



, , . .rill T^ ogy, after Butchh.) 



extend to the posterior region of the body. Branches 

 from the ganglion and from the nerve trunks extend forward and to the 

 lateral regions of the body. In the roundworms (Fig. 109) a ganglionic 

 mass, the nerve ring, surrounds the esophagus and from this mass a dorsal 

 and a ventral nerve trunk extend backwards. These are connected at 

 intervals by commissures. From the nerve ring, branches extend for- 

 ward and for a short distance backward. 



In the Annelida (segmented worms) the nervous sj'stcm is more 



