330 H. JAMES-CLARK ON THE AFFINITIES OF SPONGES. 



outline. By 2.52 p. m. (fig. 57), the posterior end of the body — at a point a little to one 

 side of the spot where it was attached to the pedicel — was also slightly indented, so that 

 in outline it presented a guitar-shaped figure, each rounded half of which bore a pair of 

 unequal cilia, and contained a contractile vesicle. In one minute more the contraction had 

 increased to such an extent that the body was divided about half way through (fig. 58). 

 By 2.54 p. m. (fig. 59), the animal had a dumb-bell shape, and the pedicel (pd) was attached 

 to one of the segments near the point of constriction. Still the process went on very 

 rapidly, and by 2.55 p. m. (fig. 60), the new bodies were widely separated, but still attached 

 to each other by a mere thread. At 3 p. m. (fig. 61), the body which was attached to the 

 pedicel was left alone, and its companion swam away to seek a new attachment, and build 

 up its stem." 



" To the last moment the hyaline envelope remained about the segments, and in fact so 

 long afterward that time and circumstances did not allow me to ascertain its final dis- 

 position. I would remark, however, that when the ovate bodies of the half-grown 

 monads (fig. 49) are contracted temporarily into a globular shape, they appear identical 

 — excepting that they lack the hyaline envelope — with these recently fissated forms. 

 In all probability, therefore, the latter lose their envelope and assume the shape of the 

 former." 



" As to the development of the stem, I think it quite certain that it grows out from the 

 posterior end of the body. The best proof of this is that I have frequently found a monad — 

 especially in the condition of the one which I described above as breaking loose from its com- 

 panion — nearly sessile upon a clean spot, and attached by a very short, faint, film-like 

 thread. From this size upward I have no difficulty in finding abundant examples as grad- 

 ually increasing in diameter as they did in length ; thus furnishing a pretty strong evidence 

 that the stem grows under the influence of its own innate powers, and is not therefore a 

 deposit emanating from the body of the monad, except, perhaps, as far as it may be nour- 

 ished by a fluid circulating within its hollow core." 



§ 12. Astasia tricophora, Clap. 



(Plate IX., figs. 45, 46). 



The transition from the mononematous Moms, Codosiga, Leucosolema, &c, to those heiero- 

 nematous FlageUata which possess at the same time a proboscidiform and a gubernacliform 

 flagellum is most aptly exemplified by that curious mimetic combination of Amoeba and Aniso- 

 nema known as Astasia tricophora,C\&^. (Trachelitis tricophorus,Ehr.). At first sight it appears 

 to be capable of all the abrupt retrogressive motions and short turnings of an Anisonema (figs. 

 65-69), without being endowed with a similar means of locomotion. One is not long, how- 

 ever, in discovering the homologue of the trail (fl 2 ) or rudder (guoemaclum) of the latter 

 in the posterior, abdominal, triangular prolongation (fig. 45, fl 2 ) of the body of the former. 

 That this is the true interpretation of the prolongation, is warranted not only by the use to 

 which it is put, as a sort of point d'appui during the amoeboid retroversions of the body, but 

 also by its persistent form whilst the animal is contorted into a shapeless, writhing mass. 

 In the midst of the paucity of distinctive topography, we are also furnished by this organ, 

 if I may so call it, with a basis of ready discrimination between the practically ventral and 

 dorsal sides of the body ; for, although it may not lie strictly in the central line of progress 



