of Edinburgh, Session 1880 - 81 . 
213 
Towards the posterior part of the foregoing region, the circular 
muscular coat becomes much thicker, while the radiating fibres 
diminish, and become of secondary importance in the greatly enlarged 
visceral area. In the succeeding part of the body, indeed, the 
muscular layers diminish in proportionate bulk, and as we approach 
the posterior end the pennate arrangement of the longitudinal 
fibres disappears, and the latter form a comparatively thin layer 
under the basement tissue — the circular coat being internal. The 
termination of the body is thus ensheathed by uniform layers of 
longitudinal and circular fibres. 
The tentacles or branchiae arise from the slightly-enlarged 
cephalic end by- a somewhat firm basal web, which is entire 
ventrally, but is widely split dorsally, so as to present the aspect of 
a double fan, as in the Sabellidae. Indeed, viewed from this 
surface, each fan or volution is supported on its own basis, which, 
moreover, is oblique — elevated in the centre, and sloping downward 
and outward externally. The basal web remains entire for about 
a third of the total length of the tentacular arrangement, and then 
breaks up into a multitude of simple, slightly-tapering filaments, 
which are pale throughout the greater part of their extent, but 
tinted of a dark brownish hue towards the tip. Externally, the 
surface of each is densely coated with cilia, which are somewhat 
longer toward the tip of the process. As ordinarily seen in the 
preparations, the branchial fan performs about three volutions. 
The skeletal elements of the apparatus commence on the sides of 
the anterior region of the body, as a series of simple longitudinal 
chitinous processes or rods — the dorsal and ventral regions at first 
being devoid of them. They lie just within the thick basement- 
membrane under the hypoderm. Proceeding distally (anteriorly) 
the chitinous skeleton becomes much more complex, from the 
fact that it develops tubes for the transmission of the circulatory 
fluid, extends round the whole circumference of the body except a 
limited region at the dorsal hiatus, and moreover splits into two or 
more rows. In the formation of the circulatory channels, the 
chitinous longitudinal processes would first seem to form a thin 
arch with two dilated pillars, and then a complete ring with much 
thinner walls. Before complete separation of the basal folds of 
the web occurs, the sections show a double row of these channels, 
