638 ORDER PERITRICHA. 



Arachnidium convolutum, S. K. PL. XXXII. FIG. 41. 



Body egg-shaped, tapering slightly posteriorly, somewhat opaque, 

 spirally furrowed or convolute throughout its length ; oral cilia nearly as 

 long as the body, more numerous but not so thick as in A . globosus ; 

 endoplast band-like, curved ; contractile vesicle spherical, subcentral. 

 Length 1-750". HAB. Salt water. 



The form and proportions of this animalcule, in addition to its considerably larger 

 size and salt-water habitat, readily distinguish this type from the preceding one. 

 A single example only has up to the present time been met with, this having been 

 obtained by the author at Bognor, on the Sussex coast, in September of the 

 year 1872. 



Arachnidium bipartitum, From. sp. PL. XXXII. FIGS. 42 AND 43. 



Body subpyriform, with a subglobose inflated posterior, and a 

 smaller but also rounded anterior portion, the constriction dividing these 

 two regions from one another bearing the tentaculate locomotive cilia, 

 these long and slender, equalling the body in length, sometimes 

 used as ambulatory organs ; parenchyma transparent, enclosing numerous 

 spheroidal, refringent corpuscles. Length 1-800". HAB. Fresh water. 



This species is briefly described by De Fromentel * as a species of Halteria, but is 

 obviously more nearly related to the present genus. The faculty it possesses of walk- 

 ing over the surface of submerged objects with the aid of its locomotive cilia is as yet 

 not known to be possessed by any other members of the Halteriadae, but is possibly 

 shared by the preceding representatives of the genus Arachnidium. The locomotive 

 cilia in this type are more slender, and evidently of firmer consistence, than in the 

 two preceding species, and would seem to possess an almost stylate character. The 

 characteristic mode of progress through the water, as depicted by De Fromentel, 

 agrees essentially with that previously noted by the author of Arachnidium globosus, 

 all the locomotive cirri being gathered together and extended in front of the oral 

 aperture, while the animalcule swims backwards after the manner of an Octopus. 

 What, during such translation, constitutes the actual motile agency has not yet been 

 ascertained, but it would seem probable that additional fine locomotive cilia are 

 developed upon the oral region. Although not represented by De Fromentel in his 

 figures, he would seem to hint, in his brief description of the present species, that such 

 fine, scarcely perceptible cilia fringe the free margin of the anterior border. 



GENUS VI. DIDINIUM, Stein. 



Animalcules free-swimming, ovate or subcylindrical, provided with 

 an anterior and posterior ciliary wreath ; the anterior extremity snout-like, 

 enclosing a tubular, protrusible, prehensile proboscis, whose distal extremity 

 is perforated by the oral aperture ; anal aperture and contractile vesicle 

 posteriorly located ; multiplying by transverse fission, and by the sub- 

 division of the endoplast into germinal elements. 



Neither of the ciliary wreaths in Didinium immediately surround the mouth or 

 take an important place in connection with that organ as in Halteria and Strombidium, 



Etudes sur les Microzoaires,' 1876. 



