328 ZOOLOGY SECT. 



probably require impregnation, and remain in an inert condition all 

 through the winter, finally developing in the spring. Most Rotifers 

 are oviparous, but some (Philodina, etc.) bring forth living young, 

 which are born by breaking through the body-wall or through the 

 cloaca, thus causing the death of the parent. 



Segmentation is total and irregular, the oosperm dividing into 

 megameres and micromeres. An epibolic gastrula is formed, the 

 blastopore closes, and invaginations of ectoderm give rise to the 

 stomodseum and proctodaeum. The tail is formed as a prolongation 

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

 an extension of the endoderrn. 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 tempera- 

 tures 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 Oligocha3te (vide Section X), or Seison on the 

 little Crustacean Nebalia. Others, again, are internal parasites, 

 such as Albertia in the crelonie of Earthworms and the intestine of 

 fresh-water Oligochsetes (Nais), and Notommata wernecJcii 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 Annulatd) is extremely close, and, in particular, the 

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

 mature trochophore with a mastax. The excretory organs recall 

 those of the Platyhelminthes, and also resemble the provisional 

 uephridia 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 Gasirotricha (Figs. 277 and 278) 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. 



