387 
of Edinburgh , Session 1873-74. 
Body-wall . — The external investment is a translucent, perfectly 
smooth, glistening cuticle, very thin on the snout, cephalic pro- 
cesses and the anterior region, but of considerable thickness and 
great tenacity throughout the rest of the body. It is this layer 
which enables such forms to bear much strain in a longitudinal 
direction, and, by its great elasticity, to dispense with a special 
circular layer of muscular fibres. In some of the Nemerteans, for 
instance, where the cutaneous tissues are soft and easily injured, a 
very perfect circular muscular coat occurs next the basement- 
membrane of the latter, and exterior to the longitudinal layer. 
When a single layer of this hyaline cuticle is examined, after 
mounting in chloride of calcium, a number of puncta, arranged with 
greater or less regularity, and apparently passing quite through it, 
are found. By tearing with needles, or examination in simple 
water, it is further seen to be composed of a closely interwoven 
series of very fine fibres, many of which have a crossed-spiral, or 
oblique direction. This is a common arrangement in such irides- 
cent forms. The cuticle readily separates from the subjacent 
layers in the preparations, a feature less evident in Ammotrypane 
and Ophelia. Beneath the foregoing is a cellulo-granular layer 
(hypoderm), which in transverse sections preserves a nearly uniform 
thickness, except inferiorly, where the nerve-cords occur. The cells 
vary in size, are filled with granules, and embedded in a hyaline 
intercellular substance. Many granules also exist amongst the 
cells. In the cephalic region a considerable thickening of the 
coat takes place, especially inferiorly, and this enlargement coin- 
cides with the diminution of the hyaline cuticular layer formerly 
mentioned. A boundary or basement-layer occurs on the inner 
surface. 
Within is a great longitudinal muscular coat, which (besides 
the passage of the oblique muscular fibres) is interrupted at two 
points in its circumference, viz., at the median line of the dorsum, 
and the opposite point inferiorly. The former is but a faint sepa- 
ration, caused by the suspensory fibres of the alimentary region ; 
the latter is a boldly-marked hiatus — the inferior fibres of the 
alimentary canal, the oblique muscular bands of the body-wall, and 
the ventral blood-vessel meeting at this point. In ordinary trans- 
verse sections this coat presents a somewhat wavy, radiated appear- 
3 n 
VOL. VIII. 
