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inclined to think that the curvature of the bifid hooklet in 
the sexual form is somewhat less acute than in the larvae, for 
so the immature forms may be called. The presence of the 
thick cuticle, which has been already mentioned, prevents 
the formation of the rugae which I noticed in my former 
paper, and which have so much the appearance of superficial 
vessels. The increased number of setae in each bundle is 
accompanied by an increased differentiation of the muscles 
moving them. A longitudinal and a horizontal muscular 
band is distinctly seen in the sexual form attached to each 
bundle (PI. XIV, fig. 4) . The bristles are frequently thrown 
into a fan-like curve, so that half have their apices directed an- 
teriorly and half posteriorly (PL XV, fig. 3). This appearance, 
no doubt, would tend to suggest the existence of a double series 
of fasciculi on each side of the mesial line, which has been 
stated to exist by Leydig in the genus Cheetogaster ; but it is 
quite clear that the bristles are really collected into but one 
bundle on each side. I have stated this in my former paper ; 
and M. Leon Vaillant, in a recent memoir on Perichaeta 
(‘ Ann. des Sciences Nat.,’ 1868), also emphatically states 
that he cannot endorse Leydig’s description. It is the more 
necessary to correct this error, since it is propagated in Victor 
Carus’s excellent e Handbuch der Zoologie.’ 
Turning our attention now to the viscera of the sexually 
mature Chsetogaster we find but little change to notice (other 
than the presence of the genitalia) . The nervous structures 
maintain the form observable in the larvae ; the remarkable 
pharyngeal or stomato-gastric ganglion and commissure, which 
is so conspicuous in Ch. diaphanus (see below) not being 
represented at all, as far as could be ascertained, even in this 
more highly developed condition. The muscular fibres pass- 
ing to the viscera from the body-wall exhibit a very definite 
nucleated structure. The stomach, or first enlargement of 
the intestine, showed very frequent undulatory or peristaltic 
contractions, which are not observed in the larvae, whilst the 
cilia of the inner wall are more obvious than in the asexual 
stage. The intestine expands again after the stomach into a 
cavity of nearly equal size, which extends as far as the sixth 
pair of abdominal fasciculi, and then contracts to a much 
smaller calibre, in which form it persists throughout the tail- 
like continuation of the body formed by the terminal somites. 
This disposition of the alimentary canal is, of course, quite 
peculiar to the sexual form, since in the larvte it is broken 
up by constrictions and new growths by the separation of 
the zooids. The perivisceral cavity contains not any, or but 
few, corpuscles other than those belonging to the genitalia. 
