SYSTEM OF AQUEDUCTS. 171 



of aqueducts to any vessels of a terrestrious animal ; but 

 before you reject the conclusion, you will consider that this 

 tribe is active only in a moist atmosphere, when too they 

 swell out, and by their greater lubricity and clearness show 

 that the body is saturated and distended with a liquid. I 

 believe that they have absorbed this liquid from the air that 

 surrounds them, the moisture, however, very probably hav- 

 ing entered through invisible pores in the skin into the 

 loose textures and very bloodvessels of the animal.* After 

 a long continuance of dry weather the land mollusca become 

 extenuated ; and retiring far within the shell, or under cover, 

 they lie exhausted and incapable of any active exertion. 



This apparatus for absorbing and containing water is dis- 

 tinguished from the circulating, and from every other sys- 

 tem of vessels by having always an outward communication 

 with the circumfluent fluid ; but the position of the external 

 orifices is Less uniform than might have been anticipated. In 

 some Gasteropods (Cypraea), there is a long slit in the sole of 

 the foot near its middle ; in orders (Haliotis) there are two 

 or three pores at each extremity ; in others (Doris, Aplysia, 

 Bulla, &c.) there are a series of orifices placed round its 

 edges. Delle Chiaie says, that where these pores exist they 

 are passages of admission to the water ; but in a large pro- 

 portion of the class the surface of the foot is imperforate, 

 and in many of these (Turbo, Trochus, Murex, f Purpuri- 

 ferae, &c.) the water enters by a peculiar orifice placed in 



of all the ducts may be easily observed when filled with mercury : M. Klea- 

 berg names them mucous ducts, but he has not been able to determine their 

 use and importance." Edinb. Journ. Nat. and Geogr. Science, ii. 63. 



* " Spallanzani found that snails absorb an abundance of water, for their 

 weight increases rapidly when they are placed in it. Jacobson has lately 

 made experiments on the absorbing power of the Vine-snail (Helix pomatia). 

 A solution of prussiate of potass, which was poured on the surface of ani- 

 mals belonging to this species, was absorbed with rapidity and passed into 

 the mass of the blood. The blood can take up such a quantity as after- 

 wards to acquire a deep blue colour, when sulphate of iron is added." 

 TIEDEMAN'S Comp. Physiology, 90. 



t In the anterior part of the foot of the Muricidse, there are to be seen 

 certain holes or antra, which are the apertures to as many little cavities lying 

 underneath, and which permeate the interior substance of the foot. There 

 are, besides, between these cavities, certain slender canals trending to the 

 same holes or antra, by means of which the whole are connected and inoscu- 

 lated together. The water then entering by the syphuncle at the will of the 

 animal, is sent upon the inferior surface of the foot, into its substance and into 

 the antra ; and flowing thence into the cavities, the foot is rendered turgid 

 and firm ; but when necessary the water, by a strong pressure, is made to 

 transude from the substance of the foot, or is spontaneously ejected when 

 life becomes feeble : the foot becomes then flaccid and extenuate. DELLE 

 CHIAIE, Anim.s. Vert. Nap. ii. 204. 



