42 Dr. T. Williams on the Mechanism of Aquatic 



enters at the moment of the diastole of the pharyngeal chamber. 

 Such a movement operates suctorially upon the fluid within the 

 sphere of its influence. Having entered the cavity, the water is 

 whirled in a thousand definite directions by the branchial cilia. 

 Every particle of material substance contained is rolled into 

 minute pellets and borne in the direction of the mouth. If it 

 be palatable, it is swallowed ; if not, it is emitted forcibly again 

 by the same siphon. The water which falls under the influence 

 of the proper branchial cilia is impelled in such manner and 

 direction, and in myriad invisible currents, that it permeates the 

 branchial membrane (fig. 1, b; fig. 2, b) by means of the meshes 

 circumscribed by the vascular bars. The passage of the water 

 through these meshes does not occur in direct currents, but in 

 streams which pass up and down the sides of the meshes several 

 times before they finally reach the intra-branchial or visceral 

 cavity — therefrom to be rejected by the intra-branchial or anal 

 siphon, so that the aerating element by this contrivance is 

 detained for some time in contact with the blood- channel. 

 The egressing current saturated with carbonic acid escapes from 

 this latter siphon in a continuous stream, — such a stream as an 

 uninterruptedly acting force alone could determine. The mi- 

 croscope was accordingly applied to the examination of the lining 

 of this siphon, anticipating the immediate detection of vigorous 

 ciliary action. Ascidians, Cynthians, and Clavellinans, submitted 

 to careful inspection, disproved the anticipation. In none, by 

 any device, could cilia be demonstrated on the inner wall of this 

 anal or intra-branchial siphon. The current, therefore, which 

 escapes at its orifice is not set in motion by any force within the 

 limit of the siphon itself, but rather by that which is placed at a 

 distance — the branchial ciliary action. The space interposed 

 between the branchial membrane and the mantle in Tunicates 

 forms a part of the intra-branchial or visceral cavity. It is filled 

 with refuse water, rendered poisonous by carbonic acid. This 

 effete fluid enacts no further part in the organism. It is finally 

 rejected. 



In the Tunicata then the two siphons are continuous through 

 the branchial stigmata. The mass of water which always more 

 or less fully distends the body of the animal, observes only one 

 normal or regular movement, viz. that tending from the extra- 

 branchial siphon (fig. 1, a) in the direction of the intra-bran- 

 chial (h). The irregular and occasional currents are propelled in 

 the reverse directions. The pharyngeal cavity may muscularly 

 contract, and now and then emit pure unrespired water, and un- 

 used alimentary substances held by this water in suspension. If 

 such discretionary power did not exist, the indiscriminating mouth 

 would swallow every solid substance borne mechanically into the 



