96 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



jacent alveoli are parallel. The acute angle is truncated, 

 and the tip of the tooth completes the triangle. The base 

 is not a straight line, but a deep, re-entrant angle, which 

 reaches nearl}^ half way to the vertex. 



(i.) Along the middle line of the alveolus a straight 

 suture marks the union of the two parts which compose it. 



(ii.) Opposite the vertex of the re-entrant angle the 

 inner end of the tooth (d) may be seen. 



(iii.) The upper or basal angles of the cluster are pro- 

 longed to form a pair of horn-like processes (e), which 

 lean towards each other and towards the axis. They are 

 immovably joined to the alveoli, although they are in reality 

 distinct pieces, or epiphyses, separated from the alveoli 

 by sutures. 



2. The dried, dark-colored remains of the concen- 

 trator muscles, which bind the parallel faces of adjacent 

 alveoli to each other. 



3. Over the points where the basal angles of the five 

 alveoli approach each other, notice the flattened, periphe- 

 ral ends of five plates, the radii (Figs. 40,/*, and 41, ^), 

 which lie on the flat inner end of the lantern. 



4. Under the ends of the five radii notice the outer ends 

 of the five radulae (Fig. 4:0, g), each of which articulates, 

 l>y a movable joint, to the basal angles of two alveoli. 



5. Make a drawing of a side view of the lantern, show- 

 ing all these points. 



6. On the inner or flat surface of the lantern notice : — 

 (i.) The axial tube for the passage of the oesophagus, 

 (ii.) The five radulfe, rectangular in a surface view, and 



with their central ends meeting around the oesophagus. 

 Notice at the outer end of each radula the notches by 

 which it articulates with the alveoli. 



(iii. ) The five radii running along the middle lines of the 



