ANNULOIDA: ROTIFERA. 183 



to be produced by the females without having access to the 

 males. 



The muscular system of the Rotifera is well developed, con- 

 sisting of bands which produce the various movements of the 

 body and foot, whilst others act upon the various viscera, and 

 others effect the movements of the jaws. 



The typical group of the Rotifera is that of the Notommatina 

 {Hydatinea of Ehrenberg). In this group the animals are all 

 permanently free, and are never combined into colonies, while 

 the integument is flexible, and the body is never encased in a 

 tube. 



Stephanoceros and Floscularia, on the other hand, are fixed, 

 and are enclosed in a gelatinous tube which is secreted by the 

 animal. Melicerta (fig. 53, B) inhabits a tubular case, which 

 the animal forms for itself by means of a special organ for the 

 purpose ; whilst Polyarthra and Triarthra are protected by a 

 stiff shell, or " lorica." 



• In Triarthra there are twelve ensiform fins, jointed to the 

 body by distinct shelly tubercles, and moved by powerful 

 muscles. These natatory organs are considered by Mr Gosse 

 to be homologous with the articulated limbs of the Arthro- 

 poda. 



In Asplanchna, whilst the masticatory organs, gullet, and 

 stomach are well developed, there is no intestine, the stomach 

 " hanging like a globe in the centre of the body-cavity," but not 

 communicating with the body-cavity. 



Affinities of Rotifera. — In their external appearance the 

 Rotifera approximate closely to the Infusoria, but the organisa- 

 tion of the former presents a very striking advance when com- 

 pared with that of the latter. Thus, in the Infusoria there is 

 no differentiated body-cavity, bounded by distinct walls, and 

 the alimentary canal is imperfect, the digestive sac simply 

 opening inferiorly into the diffluent sarcode of the centre of 

 the body. Further, there are no traces of a nervous system, 

 and the contractile vesicles, if looked upon as representing the 

 water - vascular system, are a very rudimentary form of this 

 apparatus. In the Rotifera, on the other hand, the alimentary 

 canal forms a complete tube, having an oral and an anal aper- 

 ture, and not communicating with the surrounding perivisceral 

 cavity ; and there is a well-developed nervous system, and a 

 highly complex water-vascular system. A real affinity is found 

 to subsist, however, between the Rotifera and the Planarida ; 

 both possessing external cilia, a nervous system, and a well- 

 developed water-vascular apparatus, the characters of which are 

 not dissimilar in the two groups. In the Planarida, how- 



