wrinkled. In the desiccated state rotifers in moss usu- 

 ally survive three or four years, in one presumably 

 reliable case as long as twenty-seven years. When 

 wet again they return to normal activity in from ten 

 minutes to a day. 



Reproduction in rotifers can be sexual, and the 

 sexes are separate. But much of the time the females 

 are fully in charge, producing young without having 

 to bother with males. In one small group of primitive 

 marine rotifers, so far known only from European 

 waters, males and females are nearly similar in struc- 

 ture, though the males are slightly smaller and less 

 abundant. The eggs must be fertilized, and they 

 hatch into animals of either sex. Among bdelloid 

 rotifers no males have ever been seen, and the eggs 

 laid by the females always develop, without fertiliza- 

 tion, into more females. About 90 per cent of all roti- 

 fers are members of a third group, the Monogononta, 

 in which males are produced only during a few weeks 

 of the year, at which time they are fairly abundant 

 but live for only a few hours to a few days. They usu- 

 ally impregnate the females by hypodermic injec- 

 tion through the body wall, rarely by copulation. 

 These males are often about one-third the length of 

 the female, sometimes much smaller. They are also 

 degenerate, lacking mouth and mastax, or other or- 

 gans as well. There are two kinds of females, indis- 

 tinguishable externally. During most of the year one 

 type prevails, laying eggs that develop without ferti- 

 lization into females of the same type. At critical 

 times in the year, when the environment is under- 

 going some marked change, another kind of female 

 hatches from the eggs. These are capable of being 

 impregnated by males, but if they are not, their eggs 

 hatch into males. When males do impregnate this 

 second type of female, the eggs that are laid have 

 thick, hard, and often ornamented shells and can 

 withstand drying, freezing, or other hazards. Such 

 "resting eggs" or "winter eggs" can tide the species 

 over unfavorable seasons or events; they later hatch 

 into the type of female that carries on without males. 



The Gastrotrichs 



(Phylum Gastrotn'cha) 



Anyone who examines old protozoan cultures or 

 pond debris under the microscope, looking for proto- 

 zoans or rotifers, will sooner or later see gastrotrichs, 

 elongate transparent creatures usually less than y^ci 

 of an inch long, and colorless except for any colored 

 food they have ingested. Most observers pass them 

 by as just another kind of ciliated protozoan, but the 

 cilia by which they swim or glide are restricted to the 

 under surface, and there are some on the head. 



The upper surface of the cuticle of the trunk is usu- 

 ally clothed with overlapping scales, with bristles or 

 spines, or with spined scales. Those most often seen 

 in fresh waters are bristly, have a slightly constricted 

 neck that sets off head from trunk, and end in a 

 forked tail that has at each tip a cement gland serv- 

 ing the same function as in rotifers. They browse on 

 the bottom or on vegetation, and swim only briefly. 

 About the size of rotifers, the gastrotrichs also re- 

 semble them in many details and feed on the same 

 small organisms or organic debris. They have no 

 spreading feeding disk, and food particles are sucked 

 in by a muscular throat (pharynx ) like that of nema- 

 todes, the group to which they seem to show most 

 affinity. 



About 60 per cent of known gastrotrichs live in 

 fresh waters, but one group is exclusively marine. 

 So far it is known only from European shores, where 

 the most devoted observers have worked. The ani- 

 mals glide, crawl in leech fashion, or remain attached 

 for long periods. They are hermaphroditic, produc- 

 ing both eggs and sperms. 



The group which includes most of the common 

 gastrotrichs of fresh waters has many marine mem- 

 bers also. The fresh-water forms are seldom found 

 in running water, for they favor habitats with much 

 decay, such as vegetation-choked shores of ponds 

 and lakes, mossy pools, and bogs. Surprisingly, they 

 also occur in large numbers in the damp sand near 

 the water's edge on sandy beaches. In all these gas- 

 trotrichs the male organs seem to have degenerated; 

 all individuals are females and lay eggs that develop 

 without fertilization. 



The Kinorhynchs 



{Phylum Kinorhyncha) 



A little more than a century ago, the ardent French 

 microscopist Felix Dujardin turned his attention to 

 some seaweeds collected along the coast of the Eng- 

 lish Channel. Upon these marine plants he discov- 

 ered a strange creeping animal less than ^{,; of an 

 inch long. It resembled nothing he had ever seen 

 before, and it had spines around the region he re- 

 garded as its neck. For this reason he named it an 

 "echinodere." Thirty years later, a German zoologist 

 concluded that the echinodere and several similar 

 creatures that had been discovered should be re- 

 garded as belonging to a special group, the Kino- 

 rhyncha, using this term to show that all of them pull 

 themselves along by a sort of snout. Some people 

 would have preferred the group to be called the 

 Echinodera. 



These are exclusively marine and microscopic, 

 with elongate bodies covered by a jointed cuticle that 



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