HABITS AND STEUGTURE OF ARENICOLA MARINA. 21 



7. The Gills (PI. 1, figs. 2—4). 



The general characters of these organs have been mentioned 

 in the introductory part of this paper^ and little remains to be 

 added. 



There are thirteen pairs of gills from the seventh to the 

 nineteenth chsetigerous somites inclusive. Tlie shape varies 

 from the short dendritic type of the littoral form to the delicate^ 

 richly- branched gill of the Laminarian variety. The gills are 

 hollow, being outgrowths of the body-wall enclosing an exten- 

 sion of the ca>lom, and what little evidence we have of their 

 development (see Benham, 1893) points to their being inde- 

 pendent structures, and not modified dorsal cirri. 



The walls of the gills, though thin, are muscular, and there 

 are also muscular bands stretching across the cavity of the gill 

 (fig. 33); and Milne Edwards has pointed out that the contrac- 

 tion of the gills, which often proceeds like a wave from before 

 backwards down the sides of the body, must exert a powerful 

 influence in propelling the blood partly into the efferent vessels, 

 and partly to the parietal capillaries. 



The ventral vessel supplies all the gills with their afferent 

 branches. The first seven pairs return the blood to the sub- 

 intestinal vessels, and so to the heart; while the efferent 

 branches of the remainder open into the dorsal vessel. 



8. Nervous System and Sense-organs. 



This system is composed of the brain, the oesophageal con- 

 nectives, the ventral nerve-cord, and the nerves arising from 

 tiiese. We have not been able to demonstrate a visceral 

 nervous system. 



The brain (PI. 5, figs. 25, 26) is placed in the prostomium, of 

 which it forms the chief part, being only separated from the 

 epidermis by blood-vessels lying in extensions of the coelom. 

 It is a small elongated structure, measuring "75 mm. in length 

 in ordinary shore lugs, and 1 mm. in the large "Laminarian^' 

 variety. At its anterior end the brain is divided into two stout 

 cornua {A. Cr.), separated by a cleft containing blood-vessels. 

 About the middle of the brain the cornua unite, but only for a 



