HABITS AND STRUCTURE OF AEENICOLA MARINA. 35 



and probably to the "siphon ^' of Capitellids, we have seen no 

 reason for regarding it or any other part of the alimentary 

 canal as '^respiratory '' in function. 



3. In the circulatory system the two hearts each contain a 

 cardiac body. This structure is composed of masses of granular 

 and vacuolated cells, projecting into the cavity of each ventricle. 

 Functionally they may be regarded as glandular valves pre- 

 venting the reflux of blood into the gastric sinuses. While 

 previously unknown in Arenicola, the '^cardiac body^^ has 

 been long known in allied genera (Ophelia, Trophonia, 

 Chlorhsema), but as an unpaired structure in the dorsal vessel 

 (Schaeppi, 1894). Hence, though histologically similar, it is 

 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 state- 

 ment that the latter type of <;ill 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 anterior cornua supply the 

 prostomium, the buccal papillse, 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 "■ Laminariau " variety fibrous outgrowtlis 

 from the dorsal and lateral surfaces of the brain scatter this 

 gangliouated covering. 



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



