540 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
tral stylet with its basis and the two pouches of accessory stylets, is 
formed before the young worm leaves the body of its parent. In 
these very young worms, however, the structures are much smaller 
than in the fully grown individual, the basis measuring but 0.018 to 
0.025 mm. in length, and the stylets about 0.015 mm. The basis is 
much more slender than in the adult, being only 0.004 to 0.005 mm. 
in width, and is but little larger at its posterior end than at the 
point of attachment of the stylet (pi. 25, figs. 21, 24, 25). The 
comparative diameter of the basis in an embryo and an adult worm 
is shown in figure D (p. 564). There seems to be evidence that the 
central stylet is originally formed in its definite position on the basis, 
and that it does not come from an accessory stylet, as has some¬ 
times been thought. 
Fig. A.— Transverse section through anterior portion of head, showing the highly spe¬ 
cialized frontal organ ( o.sns .) directly above the rhynchodaeum ( rhnd .); gl.ce., 
cephalic glands; ocl., ocellus; lac.sng., cephalic blood lacuna; iteg., integument; 
mb.ba., basement membrane; mu.crc., circular musculature of head; of. can.cil., 
opening of ciliated canal of cerebral sense organ, x 70. 
Correlated with the great size of the proboscis occurs an unusual 
development of the muscular walls of the proboscis sheath. These 
walls consist of an outer layer of longitudinal muscles, and an 
inner layer of circular muscles, although both circular and longitu¬ 
dinal fibers are to some extent interlaced in both layers (fig. B, p. 547). 
Body walls and integument .— The integumental glands are enor- 
