THE HABITS AND STRUCTURE OF ARENICOLA MARINA. 117 



very doubtful whether the paired structure of Arenicola, which has no 

 connection with the dorsal vessel directly, is homologous with the 

 unpaired organ of other Polychsets. 



Contrary to Wiren (1896), we regard the dorsal vessel as a distinct 

 structure, the gastric blood-system as a plexus, and we find that the 

 nephridia and body-wall, as well as the gills, are well supplied with 

 capillaries. 



4. Both the large pinnately-branching, and the smaller dendritic, 

 types of gill occur in A. marina. The usual statement that the latter 

 type of gill characterises this species, and that the former type is 

 characteristic of A. cristata, must therefore be modified. 



5. The brain is divided by a narrow cleft throughout the greater 

 part of its length. The anteria cornua supply the prostomium, the 

 buccal papillae, and give off the oesophageal nerve connectives. The 

 middle region of the brain supplies the upper part of the prostomium, 

 and the posterior cornua innervate the nuchal organ. 



In young specimens the almost uniform covering of ganglion-cells of 

 the brain is in close contact with the peculiar and complex sensory 

 epithelium of the prostomium, but in old specimens of the "Laminarian" 

 variety fibrous outgrowths from the dorsal and lateral surfaces of the 

 brain scatter this ganglionated covering. 



6. The nuchal organ, though apparently single, shows traces of a 

 double origin. It is probably an olfactory organ, and is developed 

 from the posterior region of the prostomium. 



7. The otoliths consist of quartz grains surrounded by a delicate 

 chitinoid film, as Ehlers stated. The peculiar commotion observed in 

 otocysts mounted in sea water was not noticed in others examined in 

 ccelomic fluid. Hence the motion is probably a result of diffusion 

 currents. 



8. The first pair of nephridia are in process of reduction. In the 

 others the form of the funnel at an early stage is described and figured. 

 In adult examples the terminal portions of the nephridia act as 

 receptacles for the ripe ova or spermatozoa. 



9. The specific gravity of the ccelomic fluid varies slightly, but is on 

 the average (including the corpuscles) 1 , 0288, thus being only very 

 slightly denser than sea water (l - 0264). 



10. The general analogies of Arenicola with certain other limnivorous 

 Chaetopods are very striking. With the Sipunculids the Arenicolidee 



