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



stout and thick that one need not be surprised to find Ehreuberg, in the absence of a 

 knowledge of the structure of this animalcule, mistaking the scarcely tapering flageUum for 

 the frontal prolongation of a TrackeUus. Usually it is about half again longer than the body, 

 but that of very large animals often greatly exceeds this proportion. Its mode of action, 

 as a propulsive organ, is not like that most frequently exhibited by the flagella of the truly 

 natant Ftagellata, for whilst in the latter case the vibrations pass along the whole length of 

 the cilium, in the former they are confined to its distal end, and moreover they seem to be 

 different in character ; since instead of simply undulating in a more or less restricted plane, 

 the flageUum twirls at the tip rather after the manner of a revolving helix. 



This method of progression is singularly modified by a rhombic meniscoid species of 

 Cyclidium, Duj. (non Ehr.), whose flageUum during reptation projects — -from a deeply sub- 

 terminal point of the convex side of the body — without flexure almost to its tip, and then 

 simply bends with frequent and vigorous strokes in the form of a hook, which it applies 

 sidewise against the surface over which the creature is passing, and drags it after it, tilted 

 over on one of its flanks, in a hitching, sidelong manner. 



As a tactile organ, and for the purposes of prehension, the flageUum appears, by its great 

 flexibility and vigorous action, to be eminently capable. Feeling about it with all the ap- 

 parent expectation of finally meeting with something, the animalcule keeps its proboscis in 

 a constant quiver, lashing it backward and forward in the meanwhile, or thrusting it along 

 its flanks and then abruptly withdrawing it, very much after the manner of a Lachrgmaria. 

 When a particle of food is brought near the mouth (m), it is, as it were, coaxed into it by 

 the light pulsations of the flageUum, apparently assisted by the movements of the buccal 

 margin. 



The eye-spot (s), so-called, naturally comes under consideration in connection with the 

 tactile organ. It is a very minute circular body, apparently about as broad as the diameter 

 of the flageUum, which lies a short distance behind the end of the head and just in front of 

 the mouth (to). Frequently, from its excessive faintness, and light red color, it appears 

 to be absent, but under careful scrutiny it may always be detected. The tendency which 

 prevails to undervalue the importance of this body because it is present in an apparently 

 similar position in the zoospores of Algae, no doubt hinders our advancement in the knowl- 

 edge of its true character and function. Whether it is an organ of vision of any grade, or 

 even a sensorial centre of any kind, can only be brought within the range of probability. 

 Its constant presence demands attention, and should excite inquiry on that ground alone ; 

 but when, moreover, we find it in a position which corresponds to that in which the chief 

 sensorial centres are usually situated, no mere resemblance to something else should divert 

 us into a train of fancies about the homologies of the red oil globules of the zoospores of 

 Alga, whilst the main point at issue is left in obscurity. 



If we cannot add any thing further that is positive in regard to this organ, it will be well 

 at least to attract attention to it in relation to its homologue in other Flagellata. In Phacus 

 pleuronectes, Duj., it is not a uniform red spot, but seems to be divided into two regions, one of 

 which is lunate in shape and of a bright red color, and projects forward from the upper side 

 of the other like an appendage ; whilst the main part is more deeply seated in the dorsum, 

 and consists of a colorless, but quite conspicuous, irregularly circular disc, about as broad as 

 the contractile vesicle, which it partially overlies. In this case one might, with a fair show of 

 reasonableness, suggest that the red portion alone is the true eye-spot, and that the colorless 



