IS2 ZOOLOGY 



ventral half ; the notopodium and the neuropodium respectively. 

 Each of these may bear (i.) a bundle of bristles, the setae ; 

 (ii.) in the midst of the setae, a single large bristle, the 

 aciculum; (iii.) solid fleshy prolongations of the body-wall, 

 containing a nerve, and probably tactile in function, the cirrhi ; 

 (iv.) respiratory organs, processes of the body-wall well sup- 

 plied with blood-vessels, and sometimes containing a prolonga- 

 tion of the body-cavity, the branchiae. These last are borne 

 as a rule only by the notopodium. 



The parapodia are not well developed in Arenicola. The 

 first nineteen segments bear each a small notopodium in 

 which a bundle of setae spring, and on the ventral surface a 

 small neuropodium, which bears a row of hooked bristles. 



The gills borne on the 7 th to the 19 th segments are 

 feathery branched structures, through whose thin walls the red 

 blood is visible. The tail bears no parapodia. 



Each of the segments of the body is divided into five small 

 rings : an unusual feature in Chaetopods, recalling the annula- 

 tion of the Hieudinea. 



The integument consists of the same elements as are 

 found in Lumbricus : (i.) a cuticle, (ii.) an epidermis of columnar 

 cells crowded with pigment cells, (iii.) a continuous sheath of 

 circular muscles, (iv.) a layer of longitudinal muscles, much 

 broken up by the presence of the bundles of setae, etc., and 

 (v.) a lining of peritoneal epitheliuni. 



The coelom is very spacious ; at the anterior end of the 

 body it is traversed by three septa, which mark the limits of 

 the first three segments. There are no other septa in the first 

 nineteen segments, but the tail is divided into as many 

 chambers as there are rings by vertical septa. The body is 

 further partially divided into three divisions by two longi- 

 tudinal incomplete mesenteries, which run obliquely from the 

 side of the body to near the middle ventral line. The central 

 division lodges the alimentary canal, the lateral contain the 

 nephridia. This dividing up of the body-cavity recalls the 

 arrangement in the Arehiannelids. The coelom is full of a 

 corpusculated fluid, in which, during the breeding season, the 

 ova and spermatozoa are found in great quantities. 



The alimentary canal runs in a straight line from the 



