STRUCTURE OF LOB-WORM 



259 



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 earthworm ; anteriorly and posteriorly there is 

 only one. 



The prostomial lobes are diffusely sensory, and bear also two ciliated, 

 probably olfactory, pits — the "nuchal organs." Otherwise sense 

 organs are represented only by a pair of otocyst sacs (Fig. 139), one on 

 each side of the oesophageal nerve-ring. These sacs, like those which 

 occur in many other Invertebrates, seem to have to do rather wdth 

 the direction of the animal's movements than with hearing. Professor 

 Ehlers notes an interesting series : In .-1 . claparedii there are simply 

 two open grooves ; in ^ . marina the sacs 

 have open necks, and contain foreign par- 

 ticles ; in ^. grubii and A. antillensis the 

 sacs are closed, and contain intrinsic oto- 

 liths of lime. 



^Ot. 



The 



as a 

 which 



buccal 

 '' pro- 

 grips 



Food canal. — (i) 

 cavity is protrusible 

 boscis " or introvert 

 the sand, and bears internal papillae 

 with chitinous tips. The protrusion 

 is due to the pressure of the ccelomic 

 fluid, while special muscles bring 

 about retraction. (2) The gullet 

 has smooth walls, and bears a 

 posterior pair of glands, w'hich 

 secrete a yellowish fluid, probably 

 digestive. (3) The gastric region, 

 from the heart to the twelfth or 

 thirteenth notopodium, is covered 

 many blood vessels, and has a 

 groove 



-ce.r. 



Fig. 139. — Anterior part 

 of nervous system in 

 Arenicola. — After Vogt 

 and Yung. 



c, Cerebral part on dorsal sur- 

 face ; ce.r., cesophageal ring ; 

 g., gullet ; v.n.c, ventral nerve- 

 cord ; /.«., lateral nerves; ot., 

 otocyst. 



with yellow cells and 

 median-ventral ciliated 

 (4) The intestinal region is much folded, " in a 

 concertina-hke manner," by the caudal septa, and is full of 

 sand, from which the nutritive matter has been absorbed. 

 The anus is at the very end. 



Body cavity. — This is spacious, except in the tail 

 region, and contains a viscous ccelomic fluid. Anteriorly 

 there are three transverse, partly muscular, septa or 

 diaphragms which moor the gullet. The first of these 

 diaphragms bears a pair of small pouches. Behind the 

 third diaphragm the gut swings freely until the beginning 

 of the tail region, in which there are many septa. 



