224 PHYLUM ANNELIDA. 
Skin, muscles, and appendages.—-Each segment is 
marked by about four superficial rings. ‘he epidermis 
is pigmented and secretes mucus, and is divided into 
numerous polygonal areas, separated by shallow grooves. 
Beneath the epidermis is a sheath of circular muscles, and 
then a layer of longitudinal muscles. Besides these there 
are (from the middle of the gullet to the beginning of the 
tail) thin oblique muscles arising from the sides of the 
nerve-cord, and dividing the body cavity longitudinally into 
a central and two lateral compartments. Other muscles 
control the prostomium, the proboscis, and the bristles. 
Unlike many of the marine Annelids, Avenicola has very 
rudimentary appendages. This reduction of appendages 
must be associated with the animal’s mode of life; it 
occurs also in many tube-inhabiting worms. Neither the 
prostomium nor the first segment show any trace of 
appendages, but the next nineteen have rudiments. The 
dorsal part (notopodial) consists of a tuft of bristles, whose 
bases are enclosed in a sac ;—the ventral part (neuropodial), 
separated by a short interval, bears several hooks. 
Nervous system.—This is in its general features like 
that of the earthworm, but ganglia are 
not developed. In the ventral nerve- 
cord, the ring round the gullet, and 
the slight cerebral enlargement which 
represents a brain, nerve cells occur 
diffusely scattered among the nerve 
fibres. Along the dorsal surface of the 
nerve-cord, in the branchial region, 
. there are two “giant fibres” like those 
Fic. 116.— Anterior - 5 
part of nervous sys- 19 the earthworm; anteriorly and 
tem in Avendcola.— posteriorly there is only one. 
After Vogt and Yung. 
¢., Cerebral part on dorsal The prostomial lobes are diffusely sensory, 
surface; @.r., cesoph- and bear also two ciliated, probably olfactory, 
ageal ring ; ss gullet; pits—the ‘‘ nuchal organs.” Otherwise sense 
Sordi ee ice nerves organs are represented only by a pair of oto- 
ot., otocyst. cyst sacs (Fig. 116), one on each side of the 
cesophageal nerve-ring. These sacs, like those 
which occur in many other Invertebrates, seem to have to do rather with 
the direction of the animal’s movements than with hearing. Professor 
Ehlers notes an interesting series: In 4. claparedi’ there are simply 
two open grooves ; in A. marina the sacs have open necks, and contain 
