ANATOMY AND CLASSIFICATION OF THE ARENICOLIDJl. 425 



In a paper on " The Habits and Structure of Arenicola 

 marina^' (1898) we have shown that there are on the Lanca- 

 shire coast two varieties of A. marina^ differing in habits^ 

 structure^ and times of maturity. The typical specimens of A. 

 marina inhabit the littoral zone^ living in U-shaped burrows. 

 They average about eight inches in length, and their gills, 

 which are ouly well developed in old deeply pigmented 

 specimens, are composed of 9 — 11 stems, each provided with 

 3 — 5 pairs of short lateral branches. These forms breed in 

 the summer, at any rate on the Lancashire coast. 



The second variety of A. marina is found in the upper 

 part of the Laminarian zone. It breeds in the spring, from 

 the beginning of March onwards. The gills of this form are 

 highly pinnate structures, consisting of about twelve stems, 

 united by a connecting membrane at their bases, and bearing* 

 ten or more branches on each side of the axis (cf. figs. 2 and 

 3, PI. 3, Gamble and Ashworth, 1898). This type of gill 

 has hitherto been known ouly in A. cristata. The discovery 

 of two races or varieties of A. marina distinguished by the 

 form of their gills necessitates a revision of the diagnostic 

 characters of this species, which have hitherto been based 

 chiefly on the shape of the gills and on the set^e. We were 

 unable to find the Laminarian variety during a visit in 

 September, 1897, to St. Andrews, where the coast has been 

 carefully searched for years by Professor Mcintosh and his 

 pupils without meetiug this form. It has, however, been 

 found in Jersey, and has been figured, probably from the 

 coast of Normandy, by Milne Edwards in the coloured plates 

 of Cuvier's ' Regne Animal ' (edition de Disciples, pi, viii, 

 fig. 1). From the records of A. marina made by Scandi- 

 navian and other naturalists who have examined specimens 

 from the Northern seas, it would appear that the littoral type 

 is the only one with which they were acquainted. 



The species Arenicola branchialis was established by 

 Audouin and Milne Edwards (1834, vol. ii, p. 287, and pi. 

 viii, fig, 13) . It is distinguished, according to these authors, by 

 having the first fifteen segments setigerous only, by having 



