106 LABORATORY WORK. 



With a single stroke of a sharp scalpel split the head 

 longitudinally, making the cut as nearly as possible in the 

 median plane. In the section thus made the anterior end 

 of the alimentary tract and the central part of the nervous 

 system can be easily studied. 



Just inside the mouth, which is placed in the centre of 

 the circle of arms, is the oval buccal mass, which is only 

 slightly connected with the rest of the head. In this find 

 the two horny jaws, black at the tips and shaped some- 

 thing like the beak of a parrot. Do these jaws work in a 

 vertical or in a horizontal plane? The cavity of the 

 mouth lies inside these jaws and passes nearer to the dorsal 

 jaw. Just inside the mouth-cavity is a pocket given off 

 on the ventral side, in which will be found a horny lingual 

 ribbon (radula or odontophore) , covered with minute horny 

 teeth. Could this ribbon be used in rasping the food after 

 it had passed the jaws? * Notice that the bulk of the buccal 

 mass is made up of muscles arranged to move jaws and 

 lingual ribbon. 



From the buccal mass trace the oesophagus backward to 

 the point where it was left in the previous dissection. Do 

 not cut at first in tracing it, as you would be apt to injure 

 other portions. If the section of the head be in the , 

 median plane, the course of the oesophagus will be easily 

 followed without dissection. If not, it can be traced later 

 after the nervous structures have been studied. 



A little back of the buccal mass some harder, cartilage- 

 like structures will be seen in the cut surface of the head. 

 These form a brain capsule, resembling in some respects 

 (analogous to) the vertebrate skull. In the dorsal side 



* This may be mounted as an object for the microscope by 

 taking it out, passing it through 95% and absolute alcohols, then 

 through turpentine, and mounting it on a slide in. balsam. 



