July i, 1921 
Larval Stages of H. bovis and H. lineatum 
449 
below the mouth is so variable in different specimens that it can not 
safely be relied upon for separating the species in this stage. If the 
specimen in question, however, is cleared and mounted and the cepha- 
lopharyngeal skeleton and mouth hooks, which can rarely be seen in un¬ 
cleared specimens, are examined under a medium high power of the 
microscope, the difference in the structure of the mouth hooks can readily 
be seen and serves to distinguish the species. 
In H. bovis (fig. 9-12) the upper or foremost ends of the somewhat 
shortened semicircular or crescent-shaped mouth hooks (fig. 11, B) are 
distinctly forked near the tip, and the lower or rear ends of the hooks 
(fig. 11, C) are blunt. In the illustrations (fig. 11, 15, 22) the mouth 
hooks are flattened out al¬ 
most horizontally instead of ! 1 “1-r* 
being in a nearly vertical j j j j 
plane as they are in the nor- L -I-1 — / .. 
mal larva. The articulation ^.*-1*,**^ X4 . 
of the mouth hooks with the 
cephalopharyngeal skeleton is in the case of H. bovis on the end of a 
slightly projecting knob of the skeleton turning almost at right angles 
to the axis, while in H. lineatum it occurs on the anterior tip of the skel¬ 
eton. Between the mouth hooks is a sharp spine (fig. 11; 15; 22, A) di¬ 
rected forward and slightly longer than the foremost tips of the forked 
mouth hooks. This spine is evidently used to assist in piercing or bor¬ 
ing through the tissue while the larva wanders toward the back of the 
host. After the larva reaches the back and molts to the fourth stage the 
skeleton with the mouth hooks and spine is cast off. 
The structure of the mouth hooks of the third-stage larva of H . linea¬ 
tum (fig. 13-16) is more regularly crescent-shaped, is slightly more ex¬ 
tended from end to end (fig. 
15, B, D), but is otherwise 
similar in general outline to 
H. bovis with the exception of 
^ t , being distinctly pointed in- 
Fig. 10.— Tlypodermabovts: Ventral view of third-stage larva. X 4. 
stead of forked like H. bovis 
at the anterior end (fig. 15, B) and having a strong pointed and back- 
ward-curving tooth (fig. 15, C) projecting outward about one-third of 
the length of the entire hook from the anterior tip. The lower or 
rear end (fig. 15, D) of the mouth hook of H. lineatum is also slightly 
pointed, not as much so as the forward end but considerably more 
than the rear or backward end of that of H. bovis. Out of a con¬ 
siderable number of larvae of this stage extracted from the back after the 
hide of the host had been punctured only a few specimens were found 
that showed a few very minute spines, barely visible under a high power 
of the microscope, along the anterior borders of the first few segments- 
If a larva of this stage is placed under the microscope in a vertical 
