208 Prof. H. James-Clark on the Spongiaj ciliatse 



is broadly rounded, at the sides liglitly indented and in-egu- 

 larly polygonal by mutual contact with others, and extended 

 in front into a delicate, membranous, circular collar [h b^), 

 which might be compared to a transverse section of a tube 

 which is about as long as it is broad. This collar is capable 

 of variations in form, like that of Codosiga (§ 6) and Salpin- 

 goeca {§§ 7, 8, 9), at one moment assuming a truncate conical 

 shape (figs, 43, 44, J), and in the next instant expanding its 

 distal margin into a distinct flare (fig. 42, h) which is at least 

 two-thirds as wide as the body ; or, finally, it retracts alto- 

 gether and disappears for a while, but eventually reappears 

 and expands to its fullest dimensions*. 



The Jlagellum (Jl) is the only prehensile organ which the 

 monad possesses. It arises from the middle of the frontal area, 

 and extends to a great length, at least five or six times as long 

 as the body, with scarcely any diminution in thickness. It is 

 a comparatively thick filament, and quite conspicuous, on 

 which account it is so easily seen through the whole mass of 

 the colony. It usually vibrates with considerable vigour from 

 base to tip, but occasionally assumes the quiescent state and 

 arcuate form so eminently characteristic of that of Codosiga 

 (§6), Bicosoeca (§3), and others. 



The mouth is the only organ which has not been actually 

 observed, although its position has been inferred, not only 

 from the otherwise similar structure of the monad of this crea- 

 ture to that of Codosiga (§ 6) , but because cm-rents of floating- 

 particles are constantly whirled in by the flagellum and made 

 to impinge upon the area within the collar. In addition to 

 this it may be added that more or less numerous coarse and 

 fine particles (fig. 44, d) are always present and scattered irregu- 

 larly about the interior of the monads, apparently under various 

 degrees of digestive decomposition. 



The contractile vesicles [cv) are two in number, and lie near 



* In this connexion it may be -well to mention the latest decision of 

 Carter in regard to the structure of the monociliated sponge-cell. In the 

 Annals and Magazine of Natural History, vol. xx. 1857, pi. 1. figs. 10, 

 11, this cell is represented as an oval body, with a single ciliary appen- 

 dage ; but in a subsequent communication to the same periodical (vol. iii. 

 1859, p. 14, pi. 1. figs. 12, 13, 14), a partial recantation seems to be 

 made, and the cell in question is figured with " two spines or ear-like 

 points projecting backwards, one on each side of the root of the cilium." 

 If, now, we suppose these '* two sinnes " to be the right and left profiles 

 of a membranous, cylindrical collar, such as I have described in Leucoso- 

 Unia, then it follows that the monociliated sponge-cell of Spongilla is like 

 that of the former. That Carter did not always find these " two spines," 

 may be explained by the fact that the membranous collar, as I am in- 

 clined to believe the " spines " to be, was retracted, since I have frequently 

 observed this to happen in the case oi Lencosolcnia when it was disturbed. 



