196 SEGMENT-ED WORMS OR ANNELIDA. 



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, which have on each 

 segment well-developed outgrowths or parapodia, divided 

 into a dorsal notopodium and a ventral neuropodium, 

 Arenicola 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 repre- 

 sents 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 in the earth- 

 '^"^S worm; anteriorly and posteriorly there 



in Arenicola.— After IS Only One. 



_ . , , Tne prostomial lobes are diffusely sensory, 



^urfec b e? 5£ 0B iS£ a " d *T a > so Ti "Hated probably olfactory, 



ageal ring ; g., gullet; P lts — tne nuchal organs." Otherwise sense 



v.n.c, ventral nerve- organs are represented only by a pair of otocyst 



cord; I.H., lateral nerves; saC s (Fig. 86), one on each side of the cesoph- 



'" " ! " ^ ' ageal 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 A. 



Claparedii there are simply two open grooves ; in A. marina the 



sacs have open necks, and contain foreign particles ; in A. Grubii and 



A. antillensis the sacs are closed, and contain intrinsic otoliths of lime. 



