328 Dr. Williams on the Mechanism of Aquatic Respiration. 



a broad single column (fig. 7, arrows). It is then suddenly- 

 deflected forwards into the interior of the gills {e), which, in 

 Octojms and Sepia, form hollow conical organs, in Loligo and 

 the Calamary a hollow semicylinder. In the former during the 

 moment of inspiration a copious column of water rushes up 

 the hollow axis of the gill ; in the latter families, along the 

 inferior concavity. These currents are directed by a most 

 complexly coordinated series of muscular actions. From the 

 interior of the branchiae the water is compressed by a muscular 

 power resident in the branchiae themselves. It issues in as 

 many streams as there are perforations between the ultimate 

 pinna of the organ. These streams regather themselves and 

 How backwards again in the direction of the base of the gills, 

 leaving the branchial hearts and other large blood-channels 

 situated in that region ; then driven forwards by the expiratory 

 collapse of the entire mantle, the water in form of a single 

 column enters the base of the funnel (/), through which it 

 finally leaves the cavity as an excretory and expiratory cur- 

 rent {a). 



The author will explain on another occasion how much that 

 is old and how much that is new is contained in these observa- 

 tions. In succeeding papers they will be supported by a large 

 mass of anatomical details of great interest, and hitherto, he 

 believes, unknown to naturalists. 



EXPLANATION OF PLATE IX. 



Fig. \. An imaginary vertical section through the shell and body oi Patella 

 athletica. a, water-cavity in the I'oof of the shell, capable of 

 being enlarged to the dimensions of b and c. e, gills ; f, fringed 

 edge of the mantle ; (/, g, edge of shell ; i, foot ; j, anus, termi- 

 nating in the chamber above the mouth. 



Fig. 2. A longitudinal section of the shell of Buccinum undatum, showing 

 the abdominal sjjires, a, a, a, a, which are filled with water and 

 with the abdomen of the animal, b. denotes the ligament by 

 which the animal is attached to the columella, c. 



Fig. 3 A. Male animal of Buccinum undatum taken out of its shell. At 

 b. is shown the mode in which the penis is carried in the branchial 

 chamber a. 



Fig. 3 B. The same animal with the penis withdrawn from the branchial 

 chamber. On the loft is the siphon through which, as indicated 

 by the aiTOws, the water enters the chamber {a), describing therein 

 a circuit marked by the arrows ; it escapes at the right cleft b. 



Fig. 4. The animal of Lymneus stagnulis taken out of its shell. The 

 arrows define the pulmonary chamber and the circulation per- 

 formed by the air under the vermicular action of the walls, b. is 

 the posterior limit of the thor.icic chamber; c. marks the solid 

 abdominal portion of the body, and a. the respiratory and defe- 

 catory sipiion. 



Fig. 5. Limax maximiis in outline, a. denotes the respiratory and defe- 



