224 



PHYLUM ANNELIDA. 



Skin, muscles, and appendages. Each segment is 

 marked by about four superficial rings. The 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, 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 ; tha 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, arid 

 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 

 : - ^~ earthworm; anteriorly 



FIG. 116. 

 part of nervous sys- 



and 



in the 



tem in Arenicola. posteriorly there is only one. 



After Vogt and Yung. 



The prostomial lobes are diffusely sensory, 

 and bear also two ciliated, probably olfactory, 

 pits the "nuchal organs." Otherwise sense 



., Cerebral part on dorsal 

 surface ; a?.r., oesoph 

 ageal ring ; ., gullet 

 ventral nerve 



. 

 cord ;/.., lateral nerves 



organs are represented only by a pair of oto- 

 ot. t 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 A. claparedii there are simply 

 two open grooves ; in A. marina the sacs have open necks, and contain 



