DEVELOPMENT OF ROTIFERS. 137 



constant as in Calyptraea. The tail (t) becomes conical, 

 Jarger, and the termination of the intestine and anal open- 

 ing is formed at the base. 



The internal organs are then elaborated ; first the nervous 

 system, consisting of but a single pair of ganglia arising 

 from the outer germ-layer (ectoderm). Soon after the sen- 

 sitive hairs arise on- the wings of the velum. 



Fig. 93 represents the advanced embryo, with the body di- 

 vided into segments, the pair of ciliated wings of the velum 

 (i'), and the long tail (t). At this time the shell begins to 

 form, and afterwards covers the whole trunk, but not the head. 



The inner organs are developed from the inner germ-layer 

 {endoderm), which divides into three layers, one forming the 

 middle part of the intestine, and the two others the glands 

 and ovaries. The pharyngeal jaws arise as 

 two small projections on the sides of the 

 primitive cavity. The male develops in 

 the same mode as the female. 



Though the development of the Rotifers, 

 so far as known, is more like that of the 

 mollusks than true worms, the Rotifers 

 may be regarded as a generalized cephalula 

 form, representing the larval forms of An- 

 nelids and mollusks, with decided affinities, 

 when we consider their chitinous covering ~ After Salenak y- 

 or carapace, the fold of the intestine, and the single nervous 

 ganglion, to the Polyzoa, and with more remote resemblances 

 to the Brachiopods. They are on the whole generalized forms. 

 A few species are parasitic : Albertia living internally, and 

 Balatro on the surface of the Nais-like worms. With the 

 lower Rotifers are associated a group of worm-like forms 

 represented by Chcetonotus, Ichthydium, etc., and forming 

 the group Gastrotricha. They have no mastax, and the body 

 is only ciliated near the end. Through DinophUus, a Tur- 

 bellarian worm, they are connected with the flat worms. 

 The genus Echinoderes is also regarded by Claus as a low 

 Rotifer. It seems quite apparent from this that the Rotifers 

 are a type which has originated from worms resembling the 

 generalized Turbellarian form, and which connects the latter 



