176 



THE RISE OF ANIMAL LIFE 



excretorq 

 canal 



muscle cell 



mrw cord 



Fig. 10-1. The body plan of the roundworms includes two tubes: one, the gut, within the other, the body wall. This 

 figure shows the anterior end of the worm with its three teeth surrounding the mouth, and a cross-section taken 

 anterior to the gonads. 



length of the body and terminating ante- 

 riorly in numerous ganglia. 



The body wall contains a thick muscular 

 component, separated into four banks of 

 muscle cells extending lengthwise, and so 

 attached that the animal can flex its body 

 only in a dorsal-ventral manner, a rather in- 

 effective method of locomotion in the water. 

 In fluids of high viscosity or in soil, how- 

 ever, it is more effective. 



The rather elaborate reproductive system 

 lies free in a fluid-filled cavity between the 

 body and gut wall. Since the sexes are sepa- 

 rate, only one set of organs is found in each 

 animal. Females, which are usually larger 

 than the males, possess two ovaries in the 

 shape of long coiled tubes. The two ovaries 

 continue into two oviducts, which enlarge 

 to form two uteri (singular, uterus). These 

 join to form a single short vagina, which 

 opens externally on the ventral side in the 

 anterior portion of the body. The mature 



eggs are stored in the uteri. In the male, 

 sperms are produced in a long coiled tube, 

 the single testis, which joins the vas deferens 

 and then becomes the seminal vesicle, the 

 storage place for the sperm. A pair of bris- 

 tles at the posterior end aid in conducting 

 the sperms from the male to the female dur- 

 ing copulation. The opening of the male re- 

 productive system is close to the posterior 

 end of the animal near the base of the bris- 

 tles. 



Parasitic nematodes. Because of the eco- 

 nomic significance of the parasitic nema- 

 todes we will discuss representative forms, 

 particularly those that attack man. Although 

 over 50 different species are human para- 

 sites, a still greater number affect man in- 

 directly by their ruthless destruction of his 

 domestic plants and animals. They invade 

 almost every organ of the body, their dam- 

 age depending on the kind and number of 

 individuals. Like most parasites, the nema- 



