THE HABITS AND STRUCTURE OP ARENICOLA MARINA. 89 



part of the Laminarian zone. Almost all the Arenicola 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 considerable 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 " Laminarian " 

 variety is the gill (PL VI., 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 movement of the blood. In some old specimens the 

 gills lose many of their finer branches, perhaps owing to friction or to 

 the attacks of enemies, 1 and in such cases there is an approximation 

 to the type of gill seen in the littoral variety, though a certain amount 

 of difference is always observable. 



Thus there appear to be two varieties of the common lugworm on the 

 Lancashire coast, distinguished by their habits, external features, and 

 periods of maturity, but their are no important structural points of 

 difference. 



The habits of Arenicola marina at the breeding season are still to a 

 large extent unknown, and developing eggs have not hitherto been 

 obtained. It has been stated that, w r hen mature, the animal is in the 

 habit of swimming freely (Ehlers, 1892, a), but we are unable to confirm 



1 See the curious account of the ravages of Corophium longicorne, by 

 d'Orbigny, 'Journal de Physique,' 1821. 



