172 SYSTEM OF AQUEDUCTS. 



the common passage of the rectum and matrix. From this 

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

 minal cavity, whence, hy 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 Bu^ccinum 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 sjiongy 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 flat 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 branchise 

 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 ; h, 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 ; 

 w, its termination." — Phil. Trans, for 1826, pi. xiv. fig. 3. 



