vii PHYLUM TROCHELM1NTHES 309 



more or less rod-like structures, tipped with delicate sensory hairs 

 and receiving nerves from the brain. 



Reproduction and Development. In most cases the female 

 reproductive organs have the same general character as in Brachi- 

 onus, i.e. the gonad is unpaired (ov\ consists of germarium and 

 vitellarium, and is provided with an oviduct. But in some of the 

 Bdelloida, such as Philodina, there are two ovaries, not divisible 

 into germ-gland and yolk-gland, and the oviduct is absent. The 

 males are smaller than the females and degenerate in structure, 

 the enteric canal being atrophied (Fig. 247, A). There is a large 

 testis () with a duct opening at the end of a protrusible penis (_?). 

 Apparently hypodermic impregnation sometimes takes place, i.e. the 

 body- wall of the female may be perforated at any place for the 

 entrance of the sperms. 



Three kinds of eggs are produced : large and small summer eggs, 

 which always develop parthenogenetically, the larger giving rise to 

 females, the smaller to males ; and thick-shelled winter eggs, which 

 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, &c.) 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 

 stomodaeum 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 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. 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 Oligochsete (vide infra}, 

 or Seison on the little crustacean Nebalia (Fig. 422). Others, again, 

 are internal parasites, such SisAlbertia in the coelome of Earth-worms, 

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

 werneckii 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 larva? of Annelids 

 is extremely close, and, in particular, the curious Trochospha^ra is, 



