LOWER ANIMALS 



211 



Roundworms invade all habitats from the hot 

 deserts to the Arctic Seas, and other organisms, 

 plant or animal, including fellow nematodes. 

 Members of this group, more than those of any other 

 group of animals, possess an amazing homogeneity 

 of structure and, also, some common aspects of life 

 history. Using Ascaris as an example for structural 

 features, we note first that there is a very heavy, quite 

 impervious, elastic, nonliving cuticle surrounding 

 the animal. Beneath this is a single cell layer of 

 epidermis which is organized in the adult into a con- 

 dition in which cell membranes disappear and indi- 

 vidual cells are no longer discernable. Within the 

 epidermis is a single layer of huge, longitudinal 

 muscle cells, which stretch the whole length of the 

 body. Next, there is a cavity, the pseudocoel, filled 

 with fluid. Within the cavity, the alimentary canal 

 stretches from the mouth at the anterior end to the 

 anus at the posterior end. Also lying free in the cav- 

 ity are the sex organs which communicate to the ex- 

 terior by a separate pore in the female, or by the anus 

 in the male. The excretory system consists of two cells, 

 whose main mass is centrally located; each cell is 

 connected to a diflferent longitudinal excretory canal. 

 Only half of this system may be present in other 

 forms. The nen<ous system is of the ladder type, as in 

 the Platyhelminthes, having many trunks connecting 

 anteriorly to a ring around the food pipe, or esoph- 

 agus. This nerve system is unusual in that the two 

 main trunks are dorsal and ventral rather than 

 both being lateral or ventral. Also unusual, is the 

 manner in which innervation of structures is ac- 

 complished. Instead of the nerve fibers branching on 

 the surface of the structure supplied, fibers actually 

 enter the innervated cells, running, for example, the 

 whole length of the huge muscle cells. 



The origin and function of the pseudocoel are of 

 interest. The pseudocoel is not a primary cavity, but 

 arises secondarily. The entire space is at first filled 

 with cells. Some of these cells break down and others 

 become circulatory cells or muscle cells, leaving a 

 space. Thus, the cavity is bounded on one side by 

 endodermal derivatives, the gut, and on the other by 

 mesodermal derivatives, the muscle cells. The true 

 body cavity, or coelom, which is found in entirely 

 different phyla, is completely lined by mesoderm. 



Functionally, the pseudocoel with cuticle and mus- 

 cle cells is important in locomotion. The pseudocoel 

 is filled with fluid under tension, retained by the 

 elastic cuticle. All muscle fibers are longitudinal 



and can act only to shorten the animal if all fibers 

 contract, or bend it to one side or the other if they 

 contract on one side. Motion is accomplished by a 

 series of looping actions like a flat spring, the mus- 

 cles contract on one side and are then sprung back 

 because of the compression of the body fluid by the 

 cuticle. 



Medically, one of our most important nematodes is 

 the hookworm, Necator amencanus. The adults live 

 attached to the human intestinal wall, sucking blood. 

 The eggs pass out in the excrement, and in the proper 

 soil, develop to encysted larva. Later they emerge 

 and become active again. The larvae are so sensitive 

 that the change in temperature when a barefoot per- 

 son steps on them causes them immediately to bur- 

 row into the skin. Reaching a blood vessel, the 

 larvae are carried eventually to the lung, where they 

 break out into the air space, migrate up and are 

 swallowed, eventually reaching the intestine. 



CLASS GASTROTRICHA (Fork-tailed Worms) 



Diagnosis: all microscopic; ciliate, protozoan- 

 like, but the posterior end is forked and the arched 

 top surface (the back or dorsum) has many spines; 

 found in fresh and salt water; development direct; 

 fresh water species are parthenogenetic females; 

 marine species hermaphroditic (have both male and 

 female sex organs). 



CLASS KINORHYNCHA (Spiny-crowned Worms) 



Diagnosis: 0.04 to 0.2 inch long; body of 13 or 14 

 rings, two rings form the head which is spine en- 

 circled and has a short, retractile proboscis termi- 

 nated by the mouth; the trunk rings bear lateral 

 spines; the tail region may bear long spines; ex- 

 clusively marine, known to occur in mud or sand and 

 on algae; sexes separate, development with several 

 larval stages and changes in form to adult stage. 



CLASS PRIAPULOIDEA (Club Worms) 



Diagnosis: up to 3 inches long; only three known 

 species; cylindrical, clublike, yellow or brown, 

 wormlike creatures; superficially resemble Sipun- 

 culoidea or Echiuroidea, but priapuloids have rows 

 of spines or papillae on the anterior retractile 

 proboscis and usually one or two posterior processes 

 with gill-like outgrowths; marine burrowers in bot- 



