38 W. BALDWIN SPENCEE. 



the sense-organs in connection with the two papillae close to 

 the hooks, but those of the other papillse are not so easy to 

 distinguish in sections. I have found them in the female in 

 the papillae on the ventral surface just in front of the first 

 annulus, and in the male just in front of the genital opening, 

 and probably they are present in all. No trace of them is to 

 be seen along the lateral line previously referred to. 



Undoubtedly those on the primary papillae are the most 

 strongly developed and the easiest to detect. Each has the 

 form of a little mass of modified subcuticular cells (fig. 49). 

 These are elongate, and the outer ones of the group are bent 

 round so as to form a bulb-shaped structure of more or less 

 definite shape. In other cases, as in fig. 34, which repre- 

 sents a sense-organ on the papilla above the anterior hook, the 

 bulb shape is more definite, and there is even present what has 

 the appearance of an external limiting structure which runs 

 across the cells where the nerve enters. Sometimes nuclei 

 can be detected in the cells, but more often they cannot, and 

 the cells are distinguished from those around them by their 

 forming a more darkly stained mass. In one instance I 

 detected more than one little mass on the papilla. The cuticle 

 is continued over the surface of the sense-organ, and there is 

 no trace whatever of any anterior concavity, or of stifi* pro- 

 cesses such as Stiles describes. On the primary papilla the 

 organ lies close to the openiog of the hook-gland; in fact, it 

 may here form an irregularly shaped structure of considerable 

 size. It is supplied by a special nerve which runs along by 

 the side of the duct of the hook-gland through the head region. 

 At the base of the sense-organ it swells out into a mass which 

 contains nuclei, and gives off fibres into the bases of the cells. 

 In all probability branches pass from this nerve to the other 

 papillae on the dorsal surface of the head, but they are so fine 

 that I have not been able to detect them save just where they 

 enter the base of the sense-organ. 



It appears to me impossible to speak with certainty as to the 

 nature of these sense-organs, though their constant presence, 

 and the size of the nerve supplying the one in connection with 



