MORPHOLOGY OF HIGHER TYPES 



133 



the testes are connected by means of the vasa deferentia (singular, vas 

 deferens) with the urethra, which extends from the urinary bladder 

 through the penis. There are usually accessory glands. 



Locomotor System. — Locomotor organs or organelles of a simple 

 type exist in most of the lower invertebrate animals but they can scarcely 

 be said to constitute a system. The simplest form of locomotor system 

 consists of two sets of opposing muscles. It is essential to an under- 

 standing of the operation of any locomotor system in which muscle is 

 involved to note that muscle is 

 able to contract vigorously and 

 thus exert considerable pull, but 

 that its relaxation is passive, and 

 its push negligible. A locomotor 

 system depending on muscle as the 

 motive power must therefore be 

 composed of opposed muscles. In 

 many of the worms, movement of 

 the body is brought about by the 

 alternate contraction of two main 

 sets of muscles, the longitudinal 

 and the circular. The longitudinal 

 muscles run lengthwise of the body 

 and the circular muscles encircle 

 it. If the longitudinal muscles 

 contract the body is shortened while 

 if the circular muscles contract and 

 the longitudinal muscles relax the 

 body is lengthened. Setae or 

 spines which increase the friction 

 of the body on the surface over 

 which the worm is creeping are 

 present in many worms. In the 

 leeches and in some other worms 

 of parasitic habit, muscular suck- 

 ing disks at the ends of the body, 

 or at one end and at some- other 

 point on the ventral surface, enable the worm to adhere firmly in one 

 region while the body is extended or drawn up. The alternate elonga- 

 tions and contractions of the body of these animals are accomplished, 

 however, by the contraction of two sets of muscles in the manner de- 

 scribed above. 



Levers in Invertebrates. — In most animals employing muscles, a 

 portion of the system consists of a set of levers moved by muscular con- 

 traction. Thus in each somite of the earthworm are four pairs of setae 



Fig. 99. — Urinogenital system of a female 

 mammal having a bicornuate uterus, some- 

 what schematic, bl, urinary bladder; k, 

 kidney; od, oviduct; ov, ovary; sug, urino- 

 genital sinus; ur, ureter; ut, uterus; vg, 

 vagina; *, position of embryos. {Modified 

 from WiedersheitJi . ) 



