Respiration in Invertebrate Animals. 49 



these inferences are non-sequiturs . Mr. Hancock is inaccurate in 

 affirming that all the water which enters this cavity from with- 

 out travels exclusively along the inhalent or extra-branchial 

 siphon, and never, under any circumstance, through either the 

 ventral or pedal openings *. All the water which is admitted 

 into the extra-branchial cavity is not respired; in other language, 

 does not pass through the branchiae into the dorsal or intra- 

 branchial chamber (fig. 6,/); nor is all the solid substance, which 

 it may perchance contain, seized by the mouth and swallowed. 

 The act of the passage of the true respiratory water from one 

 chamber into the other is an involuntary act. The volume of 

 the fluid and the rate of its motion are definite, and proportional 

 to the organic wants of the animal. The movement by which 

 water is drawn from without into the extra-branchial reservoir is 

 voluntary ■\ J and dependent in frequency of recurrence upon the 

 quantity of food which it may bear in suspension, and upon the 

 degree of its purity. The body of water which at any given 

 moment the extra-branchial cavity may contain, is sieved by the 

 cilia, which are distributed over the external surfaces of the 

 branchia?. These cilia, as will be subsequently explained, raise 

 a broad current (see arrows on the branchite in figs. 6, 8, & 9),' 

 very visible to the naked eye, which always and systematically 

 sets in the direction of the free or unattached borders of the 

 branchial lamellse. These currents begin at the^ attached 6r' 

 proximal edges of all the lamellse. ^ ^ 



They observe the same directions on the under as on th^ 

 upper surface of each lamella (see arrows on the branchiae in 

 fig. 7^). They are true food-searching currents. The pellets 

 formed b5r = their agency, having attained the free margin,* 



* In correcting what earnest and faithful observation and research har^ 

 convinced me to be " errors," I deal in no flattery or hypocritical circumlo- 

 cution. I do not honour great men the less because a re})etition of their 

 procedures has assured me that in some special particulars they may have 

 approved themselves false. It is because their genius has first indicated 

 a main highway through a tangled wilderness, that faithful observers^ 

 amongst their successors are enabled to mark the points whereat the sin' 

 of minor deviations from the straight course may have been committed.' 

 It is in this spirit that I have ventured to criticise the acute labours of Mr. 

 Hancock and Mr. Clark. It is in this spirit, I trust, that my criticism will 

 be received. The brief limits of these papers, in which results rather than 

 processes are embodied, preclude all reference to details, dissections, ex- 

 j)eriments, observations on the living animals, injections, &c. Once for 

 all, I affirm, that no assertion has been rashly projected in these papers 

 which has not been conscientiously submitted to the test oi fact, and 

 weighed in the balance of practical trial. 



t The influence of the cilia of the branchial lamella upon this in- 

 gressing current has never yet been clearly perceived. Such influence is 

 undoubtedly exerted. 



Ann. H^ Mag. N. Hist. Ser. 2. Vol. xiv. 4 



