Respiration in Invertebrate Animals. 41 



from structure rather than from office. In these papers accordingly 

 that siphon which opens into the pallial or ventral chamber will 

 be distinguished as the extra-branchial siphon ; that leading 

 from the dorsal, visceral or anal cavity, as the intra-branchial 

 siphon. These distinctives express only the anatomical position 

 of these tubes relatively to the branchial partition by which they 

 are separated. They involve no hypotheses. They attribute no 

 function. They cannot misguide. 



In the Tunicata the extra-branchial siphon (PI. I. fig. 1, iff j 

 2, a ; 3, a) leads into the pharyngeal cavity (b), which is homo- 

 logous with the ventral or pallial chamber of the Acephala. It 

 is the longer and higher of the two. All fluid which reaches the 

 mouth (fig. 3, b), seated at the lower boundary of this cavity, 

 must gain the pharyngeal chamber through the extra-branchial 

 siphon. All alimentary substances rejected by the mouth, that 

 is, those material particles not swallowed, are sent out again by 

 a convulsive jerk of the cavity through the same siphon. It is 

 essential to distinguish the substances thus refused by the mouth 

 from the true excrementitious pellets which are always ejected 

 by the intra-branchial siphon. 



The mode in which the surrounding element enters the pallial 

 space has distracted controvertists, and divided them in belief. 

 By Mr. Hancock, representing one class of observers, it is main- 

 tained that the inhalent current is set in motion exclusively by 

 the action of vibratile cilia seated on the lining membrane of 

 the siphon itself. By Mr. Clark this explanation is denied. 

 The former naturalist rests his theory upon the alleged demon- 

 stration of cilia on the internal surface of the inhalent siphon, 

 the latter upon observation of the currents. The inquiries 

 of Mr. Hancock were confined to the Lamellibranchiate mol- 

 lusks. But it may be stated with confidence, that what is true 

 of this class will apply to the case of the Tunicata. The dispute 

 is really easy of adjustment. The adjustment here, however, 

 fails in this sense, that the demonstration which is negative is 

 less persuasive than that which is positive. To prove a denial is 

 less easy than to substantiate an affirmation. The microscope 

 leaves it beyond doubt, that the internal lining membrane of the 

 extra- branchial siphon of the Tunicate is not provided with a 

 vibratile epithelium. They sometimes exist on the tentacles at 

 the base of the siphon, but most certainly not on the walls of 

 the latter. The water which enters this siphon is assuredly 

 therefore not drawn in by the agency of cilia within the siphon. 



Further observations are required to determine the exact 

 course of the currents excited by the cilia distributed over the 

 branchial bars. It is not proved that the water enters the 

 siphon in virtue of the cilia situated at the latter point. It 



