Species of M-onhystera. (^Nematodes). 415 



fotcil Ipn""tli 

 According to de Man^s system, the symbol « = ,. . ■ — - — ' 



„ total length , total length rr xi 



/8=T 1 — ^ , and 7 = 1 -. — ttHt- lo these 



leiigtii or oesophagus lengtli or tail 



symbols I shall, for the purposes of the present paper, add a 



„ ,, o. ^ ■ -c total length nii • 



lourtn, 6, to sienity -1^ i — : 1 \ — • inis 



' o ./ distance between anus and vulva 



can be applied, of course, only to female and perhaps, in 



some cases, to hermaphrodite individuals, but is useful in 



the present instance as a specific character. 



It should be noted that in the following descriptions the 



length of the oesophagus is taken to include the posterior 



sucking-bulb. 



1. Monhystera wilsoni, sp. n. (Figs. 1-H.) 



Measurements taken from a detailed examination of eight 

 specimens : — 



? . Length 1'4-1'6 ram. Thickness very variable, 

 especially in the region of the head and neck. 



a= 19-32-5. 



^ = 5-7-8 (usually about 7). 



7 = 8-4-10-7. 



8 = 4-4-7. 



^ unknown. 



Hab. Gill-chambers of Gecarcinus ruricola. 



The cuticle of this species is generally marked with fine 

 longitudinal sti'iations, but there are no transverse markings. 

 The head is sometimes very broad and square, sometimes 

 more tapering. The tail tapers rather rapidly from the 

 anus for about half its length ; the posterior half is more 

 cylindrical, narrowing abruptly near the tip, and ending 

 in a small conical papilla (" spinneret" or adhesive organ). 



No lateral organs are visible in the usual position near the 

 head-end. The lateral fields are about one-fiitb as wide as 

 the body. 



The mouth (fig. 2) is small, and is surrounded by six 

 slight cuticular elevations, two of which are dorsal, two 

 ventral, and two lateral. Each bears a minute setose papilla. 

 The mouth-cavity is small and shaped somewhat like a 

 "thistle funnel.'" The oesophagus is at first of the ordinary 

 cylindrical type, with thick walls and narrow lumen. At its 

 posterior end, as in M. bulbifera, de Man, there is a pear- 

 shaped sucking-bulb (fig. 3, B.), marked off from the rest of 

 the oesophagus by a constriction, and similarly distinct from 



29* 



