Respiration in Invertebrate Animals. 39 



chial vault'' (Clark), '^branchial chamber'' (Hancock), or "pal- 

 lial cavity" (Forbes and Hanley) [d). It is indifferently de- 

 scribed as the " lower," " shorter," " inferior " or " ventral." It 

 is the further of the two siphons from the hinge. Its office is 

 l" inspiratory," " inhalent," "branchial" or "prehensile." 



The " anal " siphon (fig. 7, b, V) is variously defined as the 

 " upper," " superior," " dorsal," " exhalent," " excrementitial," 

 "expiratory," "longer," &c. 



That is called " inhalent " which the most conscientious and 

 truth-loving observers declare does not inhale : that the " ex- 

 halent " to which an emissive office is strenuously denied ! There 

 are but two cavities (fig. 7, c, d) and only two siphons (fig. 6, a,b). 

 Of the latter one communicates with one cavity, the other with 

 the other. The boundaries of these cavities severally are con- 

 spicuously and unequivocally marked. They are as distinctly 

 defined as the siphons with which they respectively communicate. 

 But though clearly bounded they are not independent. Fluid 

 introduced into the one will unquestionably pass into the other*. 

 Neither the process by which food is brought to the mouth, 

 nor that of respiration, could be understood before the fact was 

 discovered of the permeability of the branchial lamellae. To Dr. 

 Sharpey should be ascribed the merit which belongs to the first 

 discovery of this point f; to Mr. Hancock that of its full and com- 



* At a subsequent stage of these inquiries, this general statement will be 

 supported by abundant evidence. — See Aeephala. 



t Dr. Sharpey's description cannot be misconstrued. *'0n removing one 

 of the valves, turning down the cloak, and putting moistened charcoal 

 powder on the surface of the gills, the finer part of the powder soon dis- 

 appears, having penetrated through the interstices of the bars or vessels 

 into the space between the two layers of the gills. On arriving there, a 

 part is often forced out again from under the border of the unattached 

 layer at the base of the gill, but most of it is convey£d rapidly backwards 

 between the two layers, and is carried out at the excretory orifice with the 

 general current . . . The coarser particles remain outside the gill and are 

 slowly carried to its edge, following the direction of the bars : they then 

 advance along the edge of the gill towards the fore part of the animal. It 

 thus appears that the water first passes in between the lobes of the mantle 

 to the external surface of the gills ; it is then/orcec? into the space enclosed 

 between their layers, from whence it is driven out at the excretory orifice, 

 to which the enclosed spaces of all the gills lead. As this process continues 

 to go on after the shell and lobe of one side are removed, it is evident that 

 the motion of the water must be mainly produced by the cilia of the gills. . , . 

 By their agei.cy the fluid is forced into the space within the gills, and this 

 operation taking place over the whole extent of the gills, must by its con- 

 centrated elFect give rise to a powerful issuing stream at the excretory 

 orifice, of which the entering stream seems to be a necessary result." — Art. 

 Cilia : Cyclop. Anat. & Phys. In this most able summaiy, three principles 

 are lucidly affirmed : — 1st. That the water concerned in breathing perme- 

 ates the branchial lamellce, and thus traverses the partition which divides 



