BY N. A. COBB. 149 



related genus, the manner of using the spear is quite different. 

 The differences will be most clearly apprehended if their considera- 

 tion be preceded by a short discussion of the mechanics of the 

 Nematode spear taken in a general sense. The office of the spear 

 is to puncture membranes which enclose the food-materials of its 

 possessor — in most cases the walls of cells. For this operation 

 it is necessary to have an opposing pull or inertia greater than 

 the force which moves the spear forward. The inertia of the 

 animal is not a sufficient reaction because of its small size 

 and consequent lightness ; therefore we find, for the production 

 of a pull, in all cases where a spear is present, well-developed 

 lips and a powerful sucking apparatus in the shape of a 

 highly muscular portion of the oesophagus specially adapted 

 to producing a partial vacuum. The lips are applied, suction 

 is then exerted, and the mouth is thus made to firmly adhere 

 to the membrane to be pierced. This force of suction is the 

 mechanical " base of operations " for the action of the spear, and 

 the pull of the suction must be greater than the force required to 

 thrust the spear forward, otherwise the lips will let go their hold 

 before the spear can accomplish its work. 



In all the genera possessed of a spear, the action of the lips 

 in obtaining a purchase is much the same, and in this respect, 

 therefore, Onyx cannot be said to present marked peculiarities. 

 When we come, however, to the manner in which the spear is 

 thrust forward, we find marked differences, and Onyx presents one 

 of the most marked types. The most emphatic morphological 

 expression of the difference existing between Onyx and its con- 

 geners is the possession by the former of a distinct muscular 

 pharyngeal bulb. There is no such bulb in any known species of 

 Tylenchus, Aphelenchus, Dorylaimus, or other spear-carrying 

 genus. In Tylenchus the spear is believed to be moved backward 

 and forward by means of muscles attached to the three chitinous 

 bulbs which constitute its posterior extremity. I believe, however, 

 that no such muscles have been observed in Dorylaimus; in fact the 

 spear in this genus appears to me often to be moved forward, not so 

 much by muscles attached to itself as by muscles attached solely 



