Respiration in Invertebrate Animals'. 53 



All the interlaminar tubes, lately so fully described by Mr. 

 Hancock, pour a constant current into this cavity, with which 

 all the intra- brauchial spaces openly communicate. Into this 

 chamber, at different points in different genera, the intestine 

 terminates (PI. II. fig. 12, b). It receives the excrementitiou ■• 

 products at once of the intestinal and branchial systems. 



This simple definition was first established in all its details by 

 the researches of Mr. Hancock. It is essentially founded upon 

 the permeability of the branchial laminae, first proved by Dr. 

 Sharpey. The correctness of this description is denied in the 

 most strenuous manner by Mr. Clark. It is impossible not 

 to admire the ardent eloquence, and sagacious inventiveness of 

 argument with which Mr. Clark defends his own views. The 

 duty of the critic, however, is sacred. The solemn sentence of 

 " error " must be pronounced alike over many of his " facts " 

 and not a few of his " inferences." 



Mr. Clark is indeed right in asserting, that frequently a 

 momentary in-current occurs through the &r-current siphon. 

 This accidental incident is utterly unimportant. The normal, 

 systematic, and necessary direction of the current in this (the 

 dorsal, upper, anal ex-current, ex-halent or m^ra-branchial) 

 siphon is centrifugal. In all Acephalans, whether siphonal or 

 non-siphonal, everything that passes through this siphon has 

 an outward tendency. The centripetal movement is irregular 

 and accidental. The stream bears far more strikingly a con- 

 tinuous character than that ingressiug at the ventral or extra- 

 branchial orifices. The uninterruptedness of this current was 

 supposed by Mr. Hancock to be due to the action of cilia lining 

 the interior of the siphon. The statement of this distinguished 

 naturalist in this particular is indisputably erroneous. This 

 siphon, like the in-current one, is not lined with vibratile epithe- 

 lium. The stream by which it is traversed is not excited by any 

 force within its own limits. 



In denying the existence of cilia within this siphon, Mr. Clark 

 is on the side of truth. But the current emanating from the 

 excurrent-siphon is continuous in character. It always goes on 

 except when the orifice is closed by voluntary muscular action. 



The continuousness of this ex-current is due, not to cilia 

 within the interior of the siphon, but to those at a distance, on 

 the branchial bars. All the cilia distributed over the internal 

 surfaces of the branchial laminae (those facing the interlaminar 

 tubes) excite currents tending in the direction of the dorsal or 

 intra- branchial cavity and that of the ex-current siphon. The 

 proofs of this interesting fact will be afterwards given *, 



* See a subsequent paper " On the Minute Structure of the Branchiae." 



