THE ANIMAL KINGDOM 409 



Example : Gordius robustus, a common horsehair worm, about 

 28 cm. long and 1 mm. wide; found in fresh water. 

 CLASS III. ACANTHOCEPHALA. There are three body 

 regions: (1) the proboscis, armed with hooks, (2) the neck, 

 (3) the trunk. The proboscis contains a ganglion from which 

 two nerve cords pass backward. There are no special sense 

 organs. There is a body cavity, but no alimentary canal. A 

 pair of nephridia opens into the reproductive duct. These 

 animals are parasitic in the intestine of vertebrates, attaching 

 themselves by the hooked proboscis. The larval stage occurs 

 in another host. Infection is by mouth from water contain- 

 ing the intermediate host (Fig. 237). 



Examples: Echinorhynchus gadi, parasitic in the cod and 

 other fishes; Macranthorhynchus hirudinaceus, parasitic in the 

 intestine of the pig; larval stage in beetle grubs. 



PHYLUM 8— TROCHELMINTHES 



Trochelminthes (wheelworms) are very small aquatic animals, 

 often microscopic in size, unsegmented, and with cilia, if present, 

 confined to the anterior end or to the ventral surface of the body. 

 CLASS I. ROTATORIA. Rotatoria (rotators), or rotifers 

 (wheel bearers), the best known members of the phylum, are 

 extremely small aquatic animals, discovered by Leeuwenhoek in 

 1703. A common species, Hydatina senta, measures only 0.6 

 mm. in length. Other species are even smaller. The body of 

 the rotifer is composed of three regions: head, trunk, and a 

 postanal tail or foot, depending upon whether the animal is free- 

 swimming or fixed. Free-swimming forms may attach them- 

 selves temporarily by means of cement secreted by a gland in the 

 tail (Fig. 238). The cuticle covering the body is secreted by the 

 epidermis (hypodermis). The head is provided with a ciliated 

 disk {corona), in the center or ventral edge of which is the mouth. 

 The cilia serve as organs of locomotion and also direct food into 

 the mouth. The alimentary canal is a straight or slightly 

 curved differentiated tube provided with an anal opening. 

 The muscular pharynx (mastax) contains on its inner surface 

 hard jaws (trophi). A pair of excretory tubes, connected with 

 flame cells, opens into the hinder end of the intestine, on either 

 side. Sometimes they open into a pulsating bladder, which in 

 turn empties into the intestine (Fig. 238, v). The central nerv- 



