:;:;r, 



H. JAMES-CLARK ON THE AFFINITIES OF SPONGES. 



portion and position, and certainly not in the intimate structure of these bodies. The whole 

 body, too, possesses a flexibility and extensibility scarcely inferior to its cilia ; at one 

 moment it is darting through the water, sharp as a lance at both ends, and at the next it is 

 as round as a ball, or worming its way through tortuous passages with every possible degree 

 of flexure short of actually tying itself into a knot." 



It would be difficult to say now whether Heteromastix belongs to the Flagellata rather 

 than to the Ciliata, or vice versa. The structure, position, and peculiar mode of action of its 

 flagella recall Anisonema (§ 13) most vividly to mind, but, on the other hand, the group of 

 cilia (cl) in the obliquely longitudinal furrow (/), in close proximity to, and evidently acting 

 more or less as allies with the flagella (fl,fl% find their parallel in the " proboscis-like lash" 

 (fig. 75,/), and vestibular cilia (cl) in the oblique buccal furrow of Pleuronema (§ 16), and 

 Dysteria (8 15). How closely allied the two latter are to the former is not the immediate 

 question here ; it is, are they related at all ? We think there can be no hesitation in reply- 

 ing in the affirmative ; but in order that the reader may have the proof before his eyes, I 

 think it will not be out of place, in this memoir, to introduce some of the undoubted Ciliata 

 which possess at the same time organs that are as truly flagellate in character as are the 

 flagella of Anisonema, Astasia, etc., etc. The genus Dysteria shall be our first example. 



§ 15. Dysteria, Huxley. (D. pror^frons, Jas.-Clk. 1 ) 

 (Plate X., figs. 77, 78.) 



This species " is an infusorian between two leaves or flexible shells (v, v 1 ), of unequal 

 width, which are united by a sort of hinge along the left border and gaping to a more than 

 equal extent along the right side, where the upper one (v) far overhangs the other (v\ bk) 

 throughout the whole length of its free edge. The broader or dorsal shell (v) is convex 

 toward the eye, and the whole organization lies within its concavity, whilst the narrower one 

 (bk, v l ) is flat, simply covering the body, and as a natural consequence does not include any 

 part of it. The open space between them is endowed with a row of closely-set, large, vibra- 

 tile cilia (cl), which differ in size according to their position ; those in front being by far the 

 longest, and those along the side scarcely more than half as long ; and in addition there is 

 one (fl) which, from its great size, has more of the character of a proboscis, or prehensory 

 flagellum, and is attached nearly at the extreme anterior border of the row " (cl). 



" It is not an easy matter in this case to determine how much of the one-sided, cilia- 

 bordered furrow corresponds to the disc or vestibule of Epistylis, Stentor, Paramecium, or 

 Pleuronema ; nor does it affect the question of the degree of obliquity of the conformation 

 of this animal, so long as we see that, whatever it may be, either wholly or in part a vesti- 

 bule, it is at least extremely oblique, and that it is not possible to view it from any point 

 but that the body appears asymmetrical in relation to it." 



" The most striking peculiarity of this creature is its habit of swinging around on a pivot 

 (fl 1 '), which consists of an ovate or lancetrshaped appendage, of considerable dimensions, 

 that projects from near the posterior end of the body, and in the line of the row of cilia. 

 The pivot possesses perfect flexibility at its base, so that the animal can move over a con- 

 siderable distance backward and forward without disturbing the point. Most of the time 

 it keeps the flat side down when gyrating around its place of attachment ; but now and then 



1 See Mind in Nature, ut sup. p. 171, fig. 100. 



