HABITS AND STRUCTURE OP ARENTOOLA MARINA. 9 



point into a minute process (PI. 3, fig. 12, proc). This 

 process is more constant in the Laminarian than in the littoral 

 variety. It appears to correspond, in position, to the charac- 

 teristic tuft of hairs on the ventral setse of the Maldanidse. 



According to the age of the specimen the ventral setae 

 differ in shape, and in the development of the toothed ridge. 

 In setse from a small specimen (17 mm. long) the apex was 

 bent more sharply on the shaft than in old examples, and the 

 teeth were very prominent (PI. 3, fig. 9). Apparently the 

 production of fresh ventral setse goes on slowly throughout 

 life, and the form which they assume before being cast out of 

 the body, varies at different ages. Their size of course varies 

 with the age of the worm to which they belong (see PI. 3), 

 but in a worm of average size their length is about '5 to 

 •8 ram. 



3. General Anatomy of the Internal Organs (PI. 2). 



In opening the body-cavity by a dorsal incision, the middle 

 part of the alimentary canal is usually forced out through the 

 cut by the pressure of the somewhat viscous cceloniic fluid. 

 Normally this portion of the canal, being longer than the 

 section of the coelom in which it lies, is swung to and fro by 

 the movements of the body. This freedom of motion is ensured 

 by the absence of mesenteries, by the absence of any vessels 

 running from the body-wall into the dorsal vessel, and by 

 the length and flexibility of the branchial and nephridial 

 vessels, which are the only connection between the stomach 

 and the body-wall. 



The ccelom is exceedingly spacious, and continuous from one 

 end of the body to the other. In front it is divided trans- 

 versely by the origins of the buccal retractors {B. Sh.), which 

 form a sheath round the proboscis, and by three septa or 

 diaphragms (Pis. 2 and 3, figs. 5 and 6). The first of these 

 septa {Dphm.) is placed obliquely, arising below behind the 

 level of the first neuropodium, and being inserted dorsally in 

 front of the first notopodial sacs. The result of this arrange- 

 ment is that between the first and second diaphragms two pairs 



