228 



THE ANIMAL KINGDOM 



and tends to hold tlie body straight if all the muscles are relaxed. Be- 

 neath the cuticle is a simple ectodermal epithelium, and beneath this a 

 single layer oi longitudinal muscle fibers. Roundworms are unique 

 among all animals in having longitudinal muscles but no circular mus- 

 cles. The only motion possible is bending of the body, which may re- 

 sult in simple curvature or in sinuous movements. Roundworms crawl 

 easily like snakes, but swim very poorly despite an extremely vigorous 

 thrashing of the body. 



The anterior mouth (Fig. 12.6) leads through a mouth cavity to a 

 muscular pharynx of unusual structure. During development the 

 pharynx arises as an ectodermal invagination together with surrounding 

 mesoderm cells, which form epithelial cells and muscle fibers respec- 

 tively. In the completed pharynx, however, the two elements are inter- 

 mingled to form a single layer of tissue. The result is a triradiate 

 pharynx, named for the shape of its lumen, which is due to the uneven 

 thickness of the wall. It is surrounded by a membrane and lined with a 

 continuation of the external cuticle. Between these are epithelial cells 

 and interspersed radial muscle fibers (Fig. 12.5). When the muscle fibers 

 contract, the lumen is enlarged, producing a sucking action at the mouth. 



89. The Vinegar Eel, Turbatrix Aceti 



A free-living nematode common in older vinegar is the vinegar eel, 

 Turbatrix aceti (Fig. 12.6), about 2 mm. long. Under the microscope 



-Mouth cavity 



Pharynx 



Nerve- rin 



PViarynx 

 bulb 



Intestine. 



Protractor 

 muscle 



Ovary 



Copula.tory 

 spicule 

 and cLnus 



Seminal 

 vesicle 



Sperm ciuct 

 Testis 



Uterus ^™--ss!^- 



Vagina 

 FEMALE MALE 



Figure 12.6. Lateral view of the vinegar eel, Turbatrix aceti. (After De Man, 1910.) 



