H. JAMES-CLARK ON THE AFFINITIES OF SPONGES. 337 



it turns upon its right edge, and performs its eccentric rotations about the appendage. This 

 is the habit which, as I said before, has impressed some observers with its similarity to the 

 Rotifera. In connection with this, too, it happens that the creature possesses a pair of 

 jaw-like, or rather pincer-like bodies (m 1 ) which lie near the entrance to the mouth, and 

 occasionally open and shut like a pair of forceps, just as similar bodies known as the jaws 

 of Rotifers do, whilst food is passing between them. Excepting the passage between these 

 jaws, there is not the least trace of an intestine, nor of any definite cavity devoted to 

 digestion. The food occupies the whole length and breadth of the body, under the same 

 circumstances as are observable in Paramecium, Pleuronema, Stentor, etc. 



" The contractile vesicles are two {cv, cv) quite small globular bodies, one of which is 

 situated just to the right of the jaws (to 1 ) and the other close to the base of the pivot (/ 2 ) ; 

 and although they contract very slowly, not oftener than once in four or five minutes, 

 they evince every characteristic, in action and physiognomy, of true infusorian, pulsating 

 vesicles. The large colorless reproductive organ (») singularly exemplifies in itself the one- 

 sidedness of the animal by its conformation to the shape of the body. One side of it is convex, 

 and, like the rest of the organization, projects into the concavity of the larger shell, whilst 

 the other face is flat and, as it were, moulded upon the plane shell. It forms a very con- 

 spicuous object just to the left of the jaws, and might easily be mistaken at first glance for 

 a contractile vesicle, especially as the true representatives of that organ are so very incon- 

 spicuous both in regard to their size and actions. 



" Now in all the organization of this animal there is nothing which is not strictly infuso- 

 rian in character. The jaw-like bodies (m l ) are not confined to this alone, for there are quite 

 a number of others which possess a similar apparatus at or near the mouth. Chilodon has 

 a complete circle of straight rods around the mouth. As for the pivot (/ 2 ), it is nothing 

 but a kind of stem, such as exists on a larger scale in Stentor, or is more peculiarly special- 

 ized in the pedestals of Epistylis, Zoothamnium, or Podophrya ; and, as counter to what we 

 see in these last, I would state that there are certain of the Vorticellians, closely related to 

 Epistylis, which have no stem whatever, and swim about as freely as Dysteria." 



§ 16. Pleuronema, Duj. (P. instabilis, 1 Jas.-Clk.) 

 (Plate X., figs. 75, 76.) 



This infusorian bears such a strong resemblance to Heteromastix (§ 14) in some of its ex- 

 ternal features, that it seems as if it might more properly have succeeded the latter in the 

 illustration of my subject ; but mere resemblances do not always indicate relationship, and 

 in the case of Pleuronema, in particular, this is most true, for it is decidedly a far more 

 highly organized animalcule than Dysteria, as we shall see by what I shall now quote from 

 an already published description. 2 



" What I wish now to show in the Pleuronema is the triple, or I might say even the quad- 

 ruple diversity of the vibrating cilia, or in other words, a quadruple specialization of one 

 type of organs, by their manifold offices ranking their possessors above those of their class 

 which attain to a less degree of complicity in this respect. The most prominent of these 

 cilia are those which are arranged in longitudinal rows (fig. 75, cl 1 ) over nearly the whole 

 extent of the body, and which most frequently are seen in a quiet state, projecting far out 



1 See Mind in Nature, ut sup. p. 148, fig. 90. 2 See rote 1. 



MEMOIRS BOST. SOC. NAT. HIST. Vol. I. Pt. 3. 9 



