i8o WORMS. 



dermis, then the circular and the longitudinal muscle- 

 fibres. • ■ ■ -u 



Appendages.— lJn\ike many of the marine Annelids which 

 have on each segment well-developed outgrowths or para- 

 podia, divided into a dorsal notopodium and a ventral neuro- 

 podium, Arenicola has very rudimentary appendages. This 

 reduction of appendages must be associated with the anirnal's 

 mode of life; the same is true of many tube-inhabiting 

 worms. The first segment has no trace of appendages, the 

 next nineteen have rudiments. The dorsal part consists of 

 a tuft of bristles, whose bases are enclosed in a sac ; — the 

 ventral part, separated by a short interval, bears several 

 hooks. 



The Nervous System 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 surfaceof the nerve-cord run two "giant-fibres" like 

 those in the earthworm. Sense-organs are represented only by 

 a pair of otocysts, one on each side of the oesophageal nerve- 

 ring. Each is a roundish sac containing fluid and calcareous 

 particles. 



Food-Canal. — The mouth is at the end of a protrusible 

 cup-like proboscis ; the gullet has smooth walls, and bears 

 an anterior and a larger posterior pair of glands which secrete 

 a yellowish fluid perhaps digestive ; the succeeding part of the 

 gut is covered with yellow cells and many blood-vessels, and 

 is divided into rings; the terminal portion is full of sand 

 from which the nutritive matter has been absorbed; the 

 anus is at the very end. 



The Body-Cavity is spacious, except in the tail region, 

 and contains a fluid. Anteriorly there are three transverse, 

 partly muscular, partitions or mesenteries which moor the 

 gullet ; in the tail region there are many such ; the median 

 part of the gut swings freely. Posteriorly there are also 

 oblique partitions which divide the segments into a median 

 and two lateral chambers. 



The Vascular System. — The blood has a bright red colour. 

 It flows forward in a dorsal vessel, running along the mid- 

 dorsal line of the gut, backward in a ventral vessel below 



