GENUS ARA CHNIDIUM. 63 7 



portion ; lorica subglobular, reaching, when the animalcule is swimming 

 and extended, as far as the annular constriction. Length i-iooo". 

 Movements like those of Halteria, sometimes swimming smoothly and at 

 others springing from side to side. HAB. Salt water. 



GENUS V. ARACHNIDIUM, S. K. 

 (Greek, arachne, spider; eidos, like.) 



Animalcules free-swimming, ovate or spherical ; oral aperture terminal, 

 central, surrounded by a circular wreath of large, flexible, tentaculiform 

 cilia, which constitute the only locomotive or prehensile organs ; endoplast 

 and contractile vesicle conspicuously developed. Inhabiting salt and fresh 

 water. 



This new genus holds a position midway between Mesodinium and Sfrombidium, 

 it having the central mouth and even oral circle of cilia characteristic of the 

 former, but wanting, as with the latter, its special supplementary leaping setae. 

 This single oral wreath of cilia at the same time attains a much greater develop- 

 ment, and assumes a character completely distinct from either of the above-named 

 genera; these cilia, indeed, resemble rather small flexible tentacula than the 

 ordinary cilia or setae of the normal representatives of this group. In their more 

 ordinary condition these organs are recurved gracefully towards the posterior 

 extremity of the body, and when in active use exhibit a perfectly independent motion. 

 The spider-like aspect of the animalcules of this genus, with their rounded bodies 

 and straggling tentacle-like cilia, has suggested the title adopted for their distinction. 

 More correctly they may perhaps be likened to the free-swimming Ccelenterate genus 

 Arachnactis. 



Arachnidium globosum, S. K. PL. XXXII. FIGS. 48 AND 49. 



Body nearly spherical, smooth, transparent : oral cilia sixteen to twenty 

 in number, thick and tentaculiform, equalling the body in length ; endo- 

 plast very large, band-like, curved ; contractile vesicle spherical, subcentral. 

 Length 1-2000". HAB. Pond water, among Conferva. 



The normal mode of motion exhibited by this species is a rolling to and fro, 

 somewhat after the manner of Strombidium, but in a more sluggish manner. 

 Occasionally it darts backwards through the water with the tentacle-like cilia collected 

 in a bundle behind in the manner indicated at PI. XXXII. Fig. 49. Under these 

 last-named circumstances it presents a fanciful resemblance to a miniature Octopus 

 swimming backwards through the water in its accustomed manner. The endo- 

 plast attains a very large relative size, closely resembling in contour that of 

 many typical Vorticellidse. On one occasion the author observed an example 

 rapidly dissolve, as though by diffluence, in the water, leaving the thick band- 

 like endoplast as the only trace of its previous existence. This, no doubt, 

 subsequently breaks up, releasing the germs of a future progeny. The few 

 examples of this minute but beautiful animalcule so far encountered were obtained 

 by the author from a pond at Stoke Newington, in February 1874, in company 

 with Dinobryon sertularia, Salpingceca amphoridium, and other Flagellate types, then 

 the subject of special investigation. The drawings of the species, as here repro- 

 duced, were accomplished with the assistance of a magnifying power of 1200 

 diameters, obtained through the use of a J^-inch object-glass by Messrs. Powell 

 & Lealand. 



