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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



side of the gullet, and, dilating slightly, opens into the side of 



the mouth-cavity. 



The digestive gland ("liver") 

 is a large brown organ making 

 up a considerable part of the 

 visceral hump. It is divided 

 into right and left lobes, of 

 which the latter (LI) is deeply 

 three-cleft, and much larger 

 than the other, which occupies 

 the final coils of the visceral 

 hump. Two ducts (bile-ducts) 

 run from the corresponding 

 lobes to the riht and left 



Refer stein).-, Mouth; Mh, mouth- 

 cavity; M, muscles; Rd, radula; 



Kn, s 

 sac 



, supporting cartilage ;Z,radular 

 ; Kf, jaw; Oe, oasophagus. 



stomach> 



f th 



left duct is extremely short, 

 and formed by the union of three branches from the subdivisions 

 of its lobe. 



The gut is lined by a single layer of epithelial cells, which are, 

 for the most part, of the simple columnar type. The radula is a 

 cuticular ribbon, developed by the epithelium lining the radular 

 sac. Upon its upper surface are numerous longitudinal rows of 

 minute pointed, backwardly-projecting "teeth." Teeth of the 

 same age are at the same level, so that a clear arrangement into 

 transverse rows is also seen. The central row contains sym- 

 metrical median teeth (undnf), while the lateral (rachidian) teeth 

 of the other rows are asymmetrical. The arrangement may be 

 expressed by the formula oo . 1 . oo , in which the 1 = the single 

 longitudinal row of median teeth and oo = the numerous longi- 

 tudinal rows of lateral teeth on each side of this. As the radula 

 is worn away in front, it grows forwards (like a finger-nail on its 

 bed) upon, and connected with, the sul-radular membrane, which 

 covers the projection from the floor of the mouth. This membrane 

 is formed by epithelium, together with underlying connective 

 tissue, and is to some extent movable upon the central part of 

 the odontophore, which is supported by two masses of gristle, the 

 odontophoral cartilages. These serve as the origins of minute 

 muscles, which are inserted into the sub-radular membrane in 

 front and behind. The cartilages consist of a clear matrix, in 

 which are imbedded numerous branched cartilage cells. The 



