172 SYSTEM OF AQUEDUCTS. 



the common passage of the rectum and matrix. From this 

 passage it rinds its way through certain pores into the abdo- 

 minal cavity, whence, by other appropriated channels, it 

 flows into the canals that ramify through the foot. In many 

 mollusks, as in the Pteropods and several Gasteropods (Te- 

 thys), no external conduits have been discovered ; and in 

 them we must believe that the water has entered within the 

 body by transudation through the skin. 



To give you a still more distinct idea of this apparatus, I 

 will quote for you the description which Mr. Ostler has 

 given of it in the Buccinum undatum, one of our commonest 

 species, and which has the power of distending the foot to a 

 size nearly, if not quite, equal to that of the shell. "A sec- 

 tion of the foot shows it to be divided into two nearly equal 

 parts, — the powerful muscle which extends from the oper- 

 culum to the spire forming the upper or posterior half, and 

 a cellular spongy mass constituting the remainder. The 

 lower surface of this portion is the disk on which the animal 

 crawls ; and, being considerably longer than the muscle, it 

 is folded upon itself, when retracted within the shell ; and 

 the operculum lies fiat above it, when it is projected and 

 extended. A transverse section of the foot, near the part 

 where it joins the body, shows four considerable tubes pene- 

 trating the spongy portion, and very near each other ; three 

 of which are in a line parallel to, and almost in contact with, 

 the muscle ; the fourth a little below the middle one of the 

 three. By a series of transverse sections of the foot, parallel 

 to the operculum we are enabled to trace these tubes ; and 

 to ascertain that they become rapidly smaller as they advance 

 until they are quite lost ; the longest of them not admitting 

 of being traced quite to the operculum. All these tubes 

 are given off at the extreme anterior point of the thorax 

 from a considerable one (Fig. 30), * which, being situated 

 under the muscular floor of this cavity, takes a direction 

 to the right side, and running just within the organs of the 



* " The animal of Buccinum undatum ; part of the spire of the branchiae 

 removed ; the mantle turned to the right side ; the upper part of the thorax 

 cut away to expose its cavity, from which the boring trunk and salivary 

 glands have been taken, a a, The foot ; b, the head; c, a kind of platform 

 raised above the floor of the thoracic cavity, on which the point of the boring 

 trunk rests, and which leads to the mouth ; d, the cavity of the thorax ; e, 

 the mantle ; J", the rectum ; g, the stomach ; h, the heart, thrown below and 

 to the right side of its natural situation, to allow the opening of the tube to 

 be seen ; i, the respiratory trunk ; k, the origins of the muscles of the boring 

 trunk ; I, the course of the tube by which the foot is supplied with water ; 

 m, its termination." — Phil. Trans, for 1 826, pi. xiv. fig. 3. 



