H. JAMES-CLARK ON THE AFFINITIES OF SPONGES. 327 



the baton upon which it spins. The largest of these twirling groups contains as many as 

 fifty fusiform bodies, but most frequently not more than half that number are grouped 

 together ; and from this they vary in decreasing numbers down to only one or two (fig. 

 48), upon each filamentous twig. In the last instances the bodies are comparatively quiet, 

 scarcely moving out of focus at each spasmodic twitch of the arcuate filament. On this 

 account, and because they offer an unobstructed view, the latter are by far the most avail- 

 able as objects for the investigation of their internal organization." 



" The relationship of the individual monads to the whole colony must, however, be studied 

 where they are more numerously congregated ; since, as will be shown presently, each 

 monad sustains a definite relation to every other one, and to the twig to which it is 

 attached." 



" Form, d-c. — The adult monads (figs. 47, 48, md) have a truncate fusiform shape, and 

 are slightly, but quite appreciably flattened on two opposite sides; so that in an end view 

 they appear to be broadly oval transversely. The attached end tapers gradually to a point ; 

 and on this account it is difficult to determine where the body ends and the twig begins. All 

 of the members of a group radiate from a common point of attachment, to which they 

 adhere by their tapering filamentous ends (fig. 48, pd 1 ). The free end is truncate, but one 

 corner of it — as if in continuation of the line along which the opposite flattened sides 

 meet — projects in the form of a rather blunt triangular beak (lp). At the inner edge of 

 the base of this beak lies the mouth (m), to which the former — as frequent observation 

 has proved — acts as a lip or prehensile organ when food is taken into the body. The 

 prevailing tint is a more or less uniform light gamboge, without the least trace of an eye- 

 spot of any color." 



"A most singular uniformity prevails in the arrangement of the several members of a 

 group. Each monad (md) is attached to its mooring in such a position that its flattened 

 sides lie parallelwise with those of its nearest neighbor ; and the beak (lp) projects from 

 that corner of the head which is most distant from the twig (pd). To give a full idea of 

 the peculiarity of this arrangement it must be stated here that the rigid, arcuate, spasmod- 

 ically twitching filament (fl) mentioned above, is attached close to the mouth (m), and in- 

 variably curves away from the beak, and consequently always toward the pedicel (pd) of 

 the colony." 



" Prehensile organs. — The only motile organs which this animalcule possesses are preemi- 

 nently prehensile in character ; and their apparent appropriation for the office of propulsion, 

 when a colony breaks loose from its attachment, I can scarcely doubt is an accidental one, 

 inasmuch as the arcuate cilium continues its spasmodic twitching without any apparent 

 deviation from its usual mode of action." 



"There are two cilia, of very unequal size, attached to the truncate end of the body. 

 The larger one of these has already been mentioned casually, as a rigid, arcuate filament (fl). 

 It does not taper, but has a uniform thickness from base to tip, and is about half again as 

 long as the body. It arises near the base of the triangular beak (lp), but appears to be 

 separated from the latter by the intervening mouth (in). When quiet it appears like a 

 bristle, and projects in a line with the longer axis of the body ; at the base bending slightly 

 toward the beak, and then sweeping off in a moderate but distinct curve in the opposite 

 direction, so that on the whole it presents a long-drawn-out sigmoid flexure. The plane of 

 this curve lies in strict parallelism with the plane of the greater diameter of the body ; in 



