294 ANIMAL BIOLOGY. [Part II. 



mass, will now be seen. Just posterior to the buccal mass, and 

 encircling the oesophagus, is the nerve-collar. Besides these organs 

 there are a number of white tubes (Fig. 87), which belong to 

 the organs of generation. We will now take the systems of 

 organs in detail. 



The Alimentary System. The student who has dissected sa' 

 far will, with care, not find much difficulty in displaying the di- 

 gestive organs as shown in Fig. 85, B. The genital organs are 

 removed, the mantle dissected from the foot, certain white glis- 

 tening bands (columella muscles) have been cut away, and the 

 whole of the foot posterior to the head removed. The -head 

 and buccal mass are shown in section, the nerve-collar not 

 being represented. The stomach, intestine, and digestive gland 

 readily assume this position when the integument is carefully 

 stripped off. 



The mouth (m.), on the dorsal side of which is the horny jaw 

 (h.j.) y leads into a buccal cavity. Into this there projects from 

 below a rounded odontophore (od.), which carries on its upper 

 surface a tooth-bearing lingual ribbon or radula. The radula is 

 developed in a diverticulum of the buccal chamber, the radula 

 sac. In this there is a layer of large cells, from each of 

 which several hard chitinous teeth are developed. Develop- 

 ment is continuous, and the whole radula grows forward over 

 the odontophore, just as our nails grow forward over their beds 

 from a groove in the skin of the finger answering to the radula 

 sac. The radula itself should be stripped off from the odonto- 

 phore of another snail, and examined under the microscope. 

 There is a central row of sharp teeth, on either side , of which 

 are a number of lateral rows. The way in which this lingual 

 ribbon is used may be seen if a pond-snail (Limnceus) be watched 

 as it licks the conferva from the sides of an aquarium tank. 



The buccal cavity leads into an oesophagus, which is soon fol- 

 lowed by an expanded crop (cr.), at the sides of which are large 

 salivary glands (s. gl.). From these the ducts (d. s. gl.) lead down- 

 wards and open into the buccal cavity above the odontophore 

 (o. s. d.). The product of the salivary gland converts starch 



