36 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



width of the proboscis, and much thicker than the length of the 

 middle chamber. 



There is great variation in the shape and size of the central 

 stylet and in the number of pouches of accessory stylets, and these 

 peculiarities are so well marked and constant that they are of much 

 importance in specific diagnoses. 



In all save the members of a single genus (Drepanophorus) 

 there is a single needle-like or conical central stylet imbedded in the 

 apex of a conical, bell-shaped or bullet-shaped basis (PI. 16, figs. 

 94, 96, 98, 100). The whole much resembles an awl with its 

 handle, and is situated in the center of the anterior border of the 

 septum, projecting forward into the cavity of the anterior chamber 

 (Text-figs. 7, 8, 12, 13, 32-34, 36-42, 47, 48, 50; PI. 17, fig. 107 ; 

 PI. 18, fig. 115; PI. 23, fig. 175; PI. 24, fig. 194; PI. 25, figs. 200, 

 201). 



The size of the central stylet and basis varies enormously in dif- 

 ferent species even of the same genus, and often bears little relation 

 to the size of the woi'm. As a rule, however, the smaller species 

 have a more minute stylet apparatus than do those of larger size. 

 Thus, in small species of Tetrastemma the basis may not exceed 

 one twentieth of a millimeter in length, and is therefore visible only 

 with considerable magnification. The largest basis yet discovered 

 occurs in An^ihiporus macracantJius (PI. 24, fig. 193 ; PI. 25, fig. 

 200) where the basis alone is about a millimeter in length and easily 

 visible to the unaided eye. In this species the basis is approxi- 

 mately one eighteenth as long as the whole body of the worm when 

 contracted. 



The stylet apparatus increases in size with the growth of the 

 individual. In embryos of Geonemertes agricola, for example, 

 when they are ready to leave the parent's body, the basis is no 

 more than one four-hundredth the volume of that in a full grown 

 individual, the comparative diameters being as one to ten and the 

 lengths as one to four or five. 



The basis is granular and often opaque in its posterior portions. 

 It is closely surrounded by a sheath of closely placed gland cells 

 which form a mold into which their secretions are poured. These 

 secretions harden and form a basis of the exact size and shape of the 

 mold. Other large glands form a wreath near the periphery of the 

 septum, their ducts leading directly to the anterior chamber of the 



