56 On the Mechanism of Aquatic Respiration. 



siphon ; b, b, extra-branchial or inhalent siphon; b, b, b, pallial or 

 extra-branchial cavity ; c, external or inferior brancbial lamella ; 

 d, internal or superior lamella ; e, membrane tying the attached 

 border of the superior branchial lamella to the side of the foot and 

 visceral mass, roofing the infra-branchial or anal chamber (f) ; the 

 arrows (g) show the currents descending from the branchiae into 

 the anal chamber, and thence out through the excurrent siphon, 

 under the membranous roof (/) connecting the branchiae of the 

 two sides ; h, mouth ; i, i, palpi. The arrows indicate the direc- 

 tion of the currents. 



Fig. 7- is a transverse ideal section of the preceding figure. It exhibits the 

 relation of the water-currents and cavities : a, incurrent siphon, 

 leading (at a') into the extra-branchial or pallial chamber d, d ; 

 b, excurrent siphon leading out of (at b') the intra-branclnal or 

 anal chamber c ; e, e, e, e, branchial lamellae. 



Fig. 7' 2 . One-half of Mytilus (fig. 10) in transverse section, showing the 

 mode in which the branchial lamellae are attached to the man- 

 tle h, h ; a, b, branchial lamellae of one side ; c, c',free proximal 

 border of respectively the upper lamella of the internal gill-plate, 

 and the lower of the external. The attached lamella (d, d!) of each 

 gill-plate are joined by a membrane at e ; g, shows the continuous 

 membrane which at the proximal borders connects the branchial 

 bars, so that at this point no water can permeate the lamella ; 

 f,f, denote the spacious grooves which receive the water from 

 the interlamellar tubes and convey it to the cloaca. The arrows 

 mark the direction of the currents. 



Plate II. 



Fig. 8. Vertical view of the common Cockle, showing the relation of the 

 branchial system to the siphons and cavities : a, excurrent siphon 

 leading out of the anal cavity (a') surrounding the visceral mass(i); 

 b, incurrent siphon conducting (at b') into the extra-branchial 

 cavity (c) ; e, arrows marking the excurrent currents coming from 

 the branchiae ; g, palpi ; f, mouth. 



Fig, 9. Ideal transverse section of the former figure : a, excurrent siphon 

 leading out of the anal cavity (e) at a' ; b, incurrent ditto opening 

 at b 1 into the general cavity of the mantle (c, c) at b' ; d, d, bran- 

 chiae ; f, the line of the mantle uniting the branchial lamellae 

 together. The arrows show the order and tendencies of the water- 

 currents. 



Fig. 10. Vertical view of Mytilus, exhibiting the systems of respiratory and 

 alimentary water-currents : a, mouth ; i, i, palps; b, extra-bran- 

 chial or pallial cavity ; c, two longitudinal halves of the rectum, 

 terminating in the excurrent channel in the mantle ; e, e &cf, sur- 

 rounded by the grooves (c) indicated by the arrows which convey 

 the expiratory water from the branchiae in the direction of the 

 cloaca ; d, d', branchial lamellae ;j, j, arrows showing the direction 

 of the currents along the distal or free border of the gills which 

 convey food to the mouth. 



Fig. 11. Ideal transverse section of the former — compare with fig. 7 2 - PL I- 

 It is designed only in outline plan to display the relative anatomy 

 of the extra-branchial (b), the branchiae and pallial membrane (rf), 

 and the intra-branchial or anal chamber (c) ; a, openings of the 

 mantle. 



Fig. 12. Oyster: a. mouth between the palpi; b, anus, emerging out of 

 the visceral mass ; c, upper half of the mantle arching over the 



