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



fact it may be said to be a direct continuation of it. It does not appear to have the charac- 

 ter of a flageOm, except when assisting the smaller cilium (f 1 ) to convey the food to the 

 mouth, and then it lays aside its rigid deportment and assumes all the flexibility and wavy 

 vibration of the prehensile organ of an Astasia." 



"The smaller cilium {f) is an excessively faint body, and almost defies the detective 

 powers of the highest objectives. This is partly due to its almost incessant activity for 

 when it is quiet, or nearly so - which happens when food is passing into the mouth - it 

 becomes comparatively quite conspicuous under a one eighth of an inch objective It is 

 scarcely as long as the greater diameter of the truncate end of the body. It arises close 

 to the base of the larger cilium (J), but whether on the right or left, or nearer or more 

 distant from the mouth than the latter, cannot be said positively. Most frequently it was 

 observed to be flexed in the same direction as its companion; and occasionally it seemed to 

 be quite evident that it was attached nearer to the mouth than the latter. It is highly 

 flexible, and vibrates with great rapidity in what appears to be a gyratory manner." ° 



" The Mouth. — This organ is never visible except when food is passing through it (figs 50 

 51, m). It then may be seen that it lies close to the beak, which acts as a sort of tip by 

 curving over the introcepted particles as they pass into the body. The mouth is highly 

 distensible ; at times allowing particles as wide as two thirds the greater diameter oAhe 

 body to pass in without any apparent extra effort (fig. 51, m). It seems undeniable that it 

 possesses discriminative powers in regard to the quality of its food. This one may readily 

 judge of for himself, by seeing the unerring precision with which the particles of floating 

 matter are thrown, by the spasmodic incurvature of the larger fiagellum, against the mouth 

 where if they are not swallowed, they are detained but for an instant by the smaller cilium' 

 quickly adjudged to be worthless, and then thrown off with a twirl of the oro- au which held 

 them in temporary abeyance. If, however, the captured morsel proves to be agreeable the 

 larger cilium (fig. 47, fl) assists the operations of the smaller one (yZ 1 ), and the lip by ab- 

 ruptly bending itself at its point of attachment and laying its basal part across the food 

 presses it into the mouth, while the terminal portion is kept in a constant wavy vibra- 

 tion, and curved toward the posterior end of the body. This is usually done in three or 

 four seconds, and then the cilia return to their usual positions, while the introcepted edi- 

 ble passes toward the centre of the body, and is there immediately enclosed in a digestive 

 vacuole (fig. 51, d). For a while the food dances about in this vacuole with a very lively 

 motion, but finally it subsides into quietude." 



« The contractile vesicle («,)._ There is a two-fold difficulty in discovering the presence of 

 this organ. _ In the first place it is comparatively quite small, and secondly, it pulsates so 

 slowly that it is very rarely possible to see it contract twice in succession between any two 

 of the abrupt, lateral deviations of the body, which the spasmodic twitchings of the arcuate 

 fiagellum produce. On this account it has not been possible to determine the precise rate 

 of its systole and diastole. It seems to contract from three to four times a minute It lies 

 near the surface, about half way between the two ends of the body, and nearly midway be- 

 tween the two extremes of its greater diameter. At the completion of its diastole it has a 

 circular outline, and appears like a clear, colorless vesicle in the midst of the yellowish tissue 

 ot the body. Upon contraction it disappears, and leaves no trace of its presence. The smtole 

 progresses slowly, as in Anisonema (A. sulcatum, Duj. ? and A. not, sp. [A. concavum, 8 131 ) 

 Cycluhum (C. not, sp.), and Phacus pleuronectes, Duj.; and in this respect contrasts strongly 



