628 DESCRIPTION OF [Rotatoria. 



pincer-like foot. Ehrenberg counted ten or eleven circular canals. 

 This animalcule is always smaller than Hydatina senta, ■which it 

 greatly resembles. Fig. 393 represents this animalcule, in which 

 the internal parts, named in the generic description, are shown. 

 Length 1-1 20th. 



Genus Hydatina. — The crystal Animalcules are destitute of eyes, 

 but have two many-toothed jaws (fig. 383*) and a fork-hke foot ; 

 locomotion is effected by the compound wheel organ, the pincer-like 

 foot, and internal muscles ; the last are most numerous in H. senta. 

 The alimentary canal has a globose oesophageal head, with four 

 muscles and jaws, and with two to five teeth. In H. senta the jaws 

 are connected by a short oesophagus to a simple conical alimentary 

 canal ; in the other species, to a constricted one. The large anterior 

 extremity? of the canal has two spherical glands. The ovarium is 

 globular. Two thin wedge-shaped glands open into a contractile 

 vesicle. The vascular system and gills are observed in H. senta. 

 In both species the central ganglia, with cervical threads or loops, 

 are visible. 



H. senta {Forticella senta, M.) — Body conical, hyaline; margin 

 of the rotatory organ ciliated ; foot truncate and robust. The 

 vibratile organ, when extended, is always in motion ; it consists of 

 a simple external wreath of cilia somewhat interrupted at the mouth, 

 and eleven internal bundles of cilia each enveloped in a muscular 

 sheath. The body has nine muscular bands, situated thus : — one 

 upper or anterior dorsal muscle (no under or posterior one), two 

 anterior ventral, and two posterior ones closing thereon ; one right, 

 and one left anterior lateral, with posterior ones in continuation. 

 The five anterior muscles arise between the muscular bundles of the 

 rotatory organ, mostly at the margin ; the dorsal ones arise from 

 the centre, near the central ganglion, and are collectively attached 

 to the internal skin of the abdomen, between the fourth and fifth 

 transverse bands, their inserted extremities being enlarged. Here 

 the four posterior muscles arise, and are inserted where the pincer- 

 like foot projects ; two longitudinally-striated muscular sheaths 

 encase the inner root of the divided foot ; and there is a sphincter 

 to the anal opening. The fibrous structure of the band-like longi- 

 tudinal muscles, as sometimes also transverse corrugations of the 



