704 Harozp HEATH, 
External features. 
In preserved material the prothorax is not so prominent as in 
other Chaetodermatidae but in living specimens the anterior eighth 
of the body is often distinctly swollen (Fig. 1) becoming at times 
almost globular especially when the animal is in the act of burrowing. 
The remaining regions are approximately of the same calibre tapering 
but slightly in the branchial portion. The superficial color of the 
body is silvery white owing to the presence of innumerable spicules 
but the abundance of dark brown granules in the liver and of food 
material in the digestive tract shining through the translucent body 
wall usually alters this to a gray or lead color. The smallest 
specimens measure 6 mm in length and 1 mm in average diameter 
while the largest was 12 mm long and 2 mm in diameter. This 
index of 1:6 is remarkably constant for all individuals. 
A dorsal sense organ is present in the form of a well developed 
groove at the posterior end of the body (Fig. 8 dso). It is surrounded 
by small spines which are absent over the sensory area itself (Fig. 13) 
though this is covered by a thin continuation of the external cuticle. 
The constituent cells are high columnar elements with sub-central 
nuclei and contain yellowish granules that become brown after 
treatment with osmic acid. It is innervated by two small branches 
of nerves from the posterior dorsal ganglion. 
The mouth, almost terminal in position, has the form of a 
vertical slit and is bounded ventrally by the buccal sensory plates 
(Mundschild) and more dorsally by small spines that cover the 
prothorax generally. The buccal-plate consists of two distinct 
triangular areas over which the cuticle is unusually thick (Figs. 2, 4). 
These plates extend half way along the sides of the mouth which 
more anteriorly is bounded by the general body surface that here 
bears somewhat larger spines than those of the immediate vicinity. 
As is noted in connection with the muscular system this region is 
very contractile, one half moving in unison with the other or 
independently of it. The extremes of the movement are shown 
in Fig 2. The underlying cells are essentially as in the genus 
Chaetoderma and are innervated by ten large nerves arising from 
as many ganglia attached to the brain. 
Beneath the mouth and buccal sensory plate there is a deep 
furrow (the halbmondförmige ventrale Grube of Wrren) that for 
want of an English equivalent may be termed the semicircular 
