HABITS AND «TRUOTURE OF ARENICOLA MARINA. S 



prostomium, proboscis, gills, aud epidermis are black. The 

 gills are better developed than those of worms living in purer 

 sand. These differences are probably due to more abundant 

 nutrition. The time of maturity of both these forms of the 

 littoral variety on the Lancashire coast is the summer, while 

 at St. Andrews they are found mature from February to 

 September. 



(2) The second variety occurs on the Lancashire coast 

 at the upper part of the Laminarian zone. Almost all the 

 Are ni col a from this zone (which can accordingly be obtained 

 only at low spring-tides) are of this kind, which when fully 

 mature, as it is from February to May, is probably one of the 

 largest Polychsets of our shores, measuring as much as fifteen 

 inches in length and three in girth. It is almost black, the 

 prostomium, proboscis, and the base of the gills being markedly 

 so. The tail is shorter in proportion to the length of the 

 body than in the littoral variety. The burrows are of con- 

 siderable length, three feet or more, and are not U-shaped, 

 but simply vertical. Like those of the littoral variety, they 

 are lined by a greenish coating of mucus. The dark " worms ^^ 

 appear to keep nearer the surface of the sand in cold weather 

 than in summer, — at least, during the winter of 1893-4 large 

 numbers were thrown up on the beach at Blackpool. 



The most distinctive character, however, of this " Lami- 

 narian^^ variety is the gill (PL 1, fig. 2), which presents a 

 structure hitherto only known in Arenicola cristata, 

 Stimps. Instead of the somewhat simple gill seen in the shore 

 lugworms, there is in the ''Laminarian'^ variety a highly 

 developed pinnate structure, consisting of about twelve 

 branches united by a connecting membrane at their bases, and 

 bearing ten or more pinnules on each side of the main axis. 

 Such a gill is undoubtedly a much more efficient respiratory 

 organ than the gill of a shore lugworm, though it does not 

 appear to possess the same power of contractility as the latter, 

 and hence probably does not contribute so much to the move- 

 ment of the blood. In some old specimens the gills lose many 

 of their finer branches, perhaps owing to friction or to the 



