\n PHYLUM TROCHELMINTHES 335 



of the postero- ventral region of the embryo, and contains at first 

 an extension of the endoderm. No metamorphosis is known to 

 take place in any member of the class. 



Ethology. — A few Rotifers live in the sea, but the majority 

 are fresh-water forms, occurring in lakes, streams, ponds, and even 

 in puddles the water of which is rendered foul and opaque by mud 

 and sewage. Frequently the water in which they live is dried up, 

 and the thick-shelled winter eggs may then be widely dispersed 

 by wind. It is even stated that the adult animals may survive 

 prolonged desiccation and resume active life when again placed in 

 water. They are able to survive prolonged exposure to tem- 

 peratures far below the freezing point of water. Many forms 

 cling to the bodies of higher animals in order to obtain a share 

 of their food, thus leading a kind of commensal existence. 

 Others go a step further and become true external parasites, 

 like Drilophaga on a fresh-water Oligochaete (vide Section X), 

 or Seison on the little Crustacean Nebalia (Fig. 457). Others, again, 

 are internal parasites, such as Albertia in the coelome of Earthworms 

 and the intestines of fresh- water Oligochsetes (Nais), and Notommata 

 u-crneclcii in the cells of the fresh-water Alga Vaucheria. 



Affinities. — The affinities of the Rotifera are very obscure. 

 Their general resemblance to the free-swimming larvae of Annelids 

 (phylum Annulata) is extremely close, and, in particular, the- 

 curious Trochosphcbra is, to all intents and purposes, a sexually 

 mature trochosphere with a mastax. The excretory organs recall 

 those of the Platyhelminthes, and also resemble the provisional 

 nephridia or head-kidneys of Annulate larvae. Lastly, the hollow 

 muscular appendages of Pedalion and Hexarthra give those genera 

 a certain resemblance — which is probably, however, merely adaptive 

 — to the nauplius or free-swimming larva of Crustacea. 



Class II. — Gastrotricha. 



The Gatfrotricha (Figs. 272 and 273) are a small group of minute fresh-water 

 animals, which are apparently allied, though certainly not very closely, to the 

 Rotifera, and are on that account placed in the present phylum. The body is 

 spindle-shaped with flattened ventral surface. The ventral surface bears two 

 longitudinal bands of cilia ; the dorsal is non-ciliated, but in some forms bears a 

 number of longitudinal rows of slender, pointed, cuticular processes. The aboral 

 end is narrow and usually bifurcated. 



On the head are four tufts of flagella, which are partly sensory, partly 

 vibratile. The mouth, situated at the anterior end, leads by a narrow tube into 

 the thick-walled oesophagus. At the beginning of the latter are a number of 

 small chitinous denticles, and in front of them a circlet of set*. The oesophagus 

 leads to a wide elongated stomach followed by a short intestine which terminates 

 in an anal aperture at the posterior extremity. The nephridia are a pair of 

 unbranched coiled tubes each opening on the ventral surface and terminating 



