their Respiration. 107 



unadulterated before the aperture is shut. This process is 

 not so obvious in the branchial Mollusca, and in many of 

 them, from the position of the gills, such a function is not 

 necessary to renew the water around them. Where, how- 

 ever, the gills are strictly internal, it seems probable that the 

 water is regularly changed when the creatures are in their 

 natural habitats and undisturbed : we know that such is the 

 case with the Cephalopoda, in which inspiration and ex- 

 spiration are well marked. " The first is effected by a gradual 

 dilatation of the sac in every direction, but particularly at the 

 sides, accompanied by a subsidence of the lateral valves, col- 

 lapse of the walls of the funnel, and a rush of water through 

 the lateral openings into the sac. Inspiration being com- 

 pleted, the lateral valves are closed, the sac is gradually 

 contracted, the funnel erected and dilated, and the water ex- 

 pelled through it with great force, and in a continued stream." 

 Dr. Coldstream, from whose letter I quote the preceding 

 sentence, has seen the stream emitted by an individual of the 

 Octopus ventricosus, " whose sac measured about four inches 

 in length, carry light bodies to the distance of eleven inches 

 from the orifice of the funnel. Respiration is performed 

 more frequently in young than in adult individuals. One, 

 whose sac measured 1^ in. in length, I saw respire 18 times 

 per minute ; and the larger one, mentioned above, respired 

 10 times per minute. The time seemed to be pretty equally 

 divided between inspiration and exspiration." In the bi- 

 valves, whose cloak forms a shut sac, the water is sucked in 

 through the siphonal tube, when the capacity of the sac is 

 increased by its own expansion, or by the opening of the 

 shells ; and by its muscular contraction, aided sometimes by 

 the closure of the shells, it is again expelled in a stream from 

 the anal siphon : but there is no regularity in the process in 

 such species as I have observed in confinement. It is the 

 same with the Mollusca tunicata. The branchial sac is 

 muscular, and just as its capacity is enlarged, apparently by 

 the contraction of its longitudinal fibres, the water flows in 

 to fill the space in a slow and uniform current, through the 

 branchial aperture only, for none can be detected entering by 

 the anal orifice. It is, after a space, expelled again by a con- 

 traction of the annular fibres of the sac, but the voluntary 

 contractions for this purpose, as stated above, take place at 

 irregular intervals of time, and, for the most part, not 

 oftener than once in a minute. {Cu\\e\\Molliis. Me7n. xx. p. 17.; 

 Coldstream, in Edin. New Phil. Joiirn. for July, 1830, p. 240.) 

 I have told you that the respiration of the Mollusca is at 

 all times slow, and easily suspended for a long })eriod ; but, to 

 obviate the inconveniences which might result from this, and 



