C. L. Boulenger 
145 
The central portion of the nervous system can be seen in many 
individuals and takes the form of a ring encircling the oesophagus at 
a point just anterior to the level of the excretory opening. 
The genital rudiment is quite conspicuous, forming a lens-shaped 
body situated ventrally to the intestine and opposite the fifth intestinal 
cell. Under a high magnification it can be seen to consist of a number 
of cells (usually about 8-12) with conspicuous nuclei. 
The Biology of the “ Ensheathed ” Larvae. 
The free larvae are extremely active and in a liquid medium at 
laboratory temperatures (13°-15° C.) perform vigorous, snake-like 
movements similar to those described in the mature larvae of other 
Fig. 2. Posterior end of body of “exsbeathed” larva, viewed from right side and from 
dorsal side. a. anus, x 750. 
Strongylid worms. When placed in a watch-glass or tube containing 
water they immediately sink to the bottom and are evidently incapable 
of keeping afloat by swimming movements. 
The larvae of Nematodirus filicollis possess the same migratory 
instincts as the larvae of Haemonchus contortus and Anchylostoma 
duodenale and are able to climb vertical surfaces if these are kept 
sufficiently moist. The majority of my cultures were kept in a moist 
chamber in small glass dishes about 40 mm. in diameter and with walls 
25 mm. high, the larvae as they hatched during warm weather immedi¬ 
ately commenced to ascend the sides of the dishes and after a few 
