PHYLA NEMATHELMINTHES, NEMATOMORPHA, AND ACANTHOCEPHALA 



151 



Ascaris lumbricoides— a 

 roundworm parasitic in man 



Anatomy 



This is the common roundworm parasitic 

 in the intestine of man. The sexes are sepa- 

 rate. The female is the larger and measures 

 up to 16 inches in length and to V4 inch 

 in diameter. The body has a dorsal and 

 ventral, narrow white line, running its en- 

 tire length, and a broader lateral line on 

 either side. The tough cuticle is smooth and 

 marked with fine striations. The mouth 

 opening is in the anterior end and is sur- 

 rounded by one dorsal and two lateroven- 

 tral lips. In the male, near the posterior end, 

 is the cloacal opening, from which extend 

 two chitinous rods, the penial spicules, of 

 use during copulation. Many ventral, prea- 

 nal, and postanal papillae are also present 

 in the male. The male is considerably 

 smaller and more slender than the female; 

 one of its best distinguishing characteristics 

 is the sharply curved posterior end. In the 

 female, the vulva or genital pore is located 

 ventrally at about one-third the length of 

 the body from the anterior end. 



The body contains a straight digestive 

 tract and other organs (Fig. 78). Between 

 the intestine and the body wall is a body 

 cavity, which is called a pseudocoel. It is 

 not a true coelom because there is no true 

 mesodermal epithelium covering the intes- 

 tine. The digestive tract is very simple. A 

 small mouth cavity opens into the mus- 

 cular esophagus, or pharynx, which is from 

 10 to 15 mm. long. The esophagus draws 

 fluids from the intestinal contents of the 

 host into the long nonmuscular intestine, 

 and the nutriment is absorbed through the 

 walls. The posterior portion of the intestine 

 is known as the rectum in the female, which 

 discharges through the anus. But in the male 

 the intestine and reproductive system open 



Figure 78. Facing page, female Ascaris. Side view 

 of a specimen on left, and a dissection on the right 

 to show the internal organs. 



into a common passage way, the cloaca, and 

 the opening to the outside probably should 

 be called a cloacal opening; however, it is 

 usually termed an anus. 



The excretory system consists of two 

 longitudinal tubes, one in each lateral line 

 (Fig. 79); these open to the outside by a 

 single excretory pore situated near the 

 anterior end in the midventral body 

 wall. 



A ring of nervous tissue surrounds the 

 esophagus and gives off two large nerve 

 cords, one dorsal, the other ventral, and a 

 number of other smaller strands and connec- 

 tions. 



Reproduction 



The male reproductive organs are a single, 

 coiled, threadlike testis, from which a vas 

 deferens leads to a wider tube, the seminal 

 vesicle; this is followed by the short, mus- 

 cular, ejaculatory duct which opens into 

 the cloaca. In the female (Fig. 78) lies a 

 Y-shaped reproductive system. Each branch 

 of the Y consists of a coiled threadlike 

 ovary, which is continuous with the 

 oviduct and uterus. The uteri of the two 

 branches unite into a short muscular tube, 

 the vagina, which opens to the outside 

 through the vulva. Fertilization takes place 

 in the oviduct. The egg is then surrounded 

 by a thick, rough-surfaced shell, and passes 

 out through the vulva. The genital tubules 

 of a female worm may contain as many as 

 27 million eggs in various stages of develop- 

 ment at one time, and each mature female 

 lays about 200,000 eggs per day. 



The eggs of the ascaris are laid inside of 

 the intestine of the host and pass out in the 

 feces. They are very resistant; if deposited 

 on the soil they may remain alive for many 

 months. Embryos are formed, under favor- 

 able conditions, in about 14 days. Infection 

 with the ascaris results from ingesting eggs 

 containing embr^'os. The eggs are usually 

 carried to the mouth with either food or 

 water, or by accidental transfer of soil con- 

 taining them. They do not regularly hatch in 

 the stomach but pass on to the small intes- 



