THE HABITS AND STRUCTURE OF ARENICOLA MARINA. 91 



achaetous portion (Fig. 1, MET), which in many specimens appears to 

 be composed of fonr annnli, divided, however, by secondary circular 

 markings. The first chsetigerous annulns is produced into a strongly 

 marked ridge, just behind which the notopodial setse (Chn. 1 ) are 

 inserted, the corresponding neuropodia (N'm. 1 ) being very short and 

 containing only a few seta?. The intervals between the chsetigerous 

 annnli are subdivided into rings, of which there are, in the "Lami- 

 narian" variety, 2 2 4 4 4..., and in the littoral variety 23444 

 . . . respectively. 



The chsetigerous annnli do not mark the true somites into which the 

 body is divided. From a consideration of the internal anatomy (see p. 

 94) we have reasons for believing that, in the middle region of the 

 body, the second groove behind each chaetigerous annulus marks the 

 boundary between the somites. A somite is, therefore, composed of a 

 chaetigerous annulus together with three annnli in front of, and one - 

 behind, it. The parapodia are not situated at the beginning, but 

 slightly behind, the middle of the somites to which they belong, thus 

 confirming Benham's observations on the postdarval stage (1893). 



The anterior region of the body is thus composed of the prostomium, 

 six chsetigerous somites, and a region between these, made up probably 

 of two somites, but the exact number is somewhat doubtful. (See 

 Plate VL, Fig. 1, and explanation, p. 120.) 



The second or branchial region of the body is composed of thirteen 

 somites, and is distinguished by the presence of gills, a pair of which 

 are attached to a slight fold of the skin just behind the notopodia. 

 The first gill is A T ariable, usvially fairly well developed, but always 

 smaller than the rest and sometimes absent. The gills about the 

 middle of the branchial region are frequently, but not always, the 

 largest. Both the gills and notopodia are very sensitive, and are 

 retracted from time to time on the application of stimuli, such as a 

 strong light. This contraction of the gills proceeds sometimes as a 

 wave down the body, and as Milne Edwards (1838) pointed out in his 

 classical paper, considerably assists the circulation of the blood. The 

 neuropodia in the branchial region extend towards the mid-ventral 

 line, so as almost to meet, and are only separated by a groove which 

 marks the line of the nerve cord. This groove is continued on to the 

 prostomium by a pair of diverging arms ("Metastomial grooves") 

 underlying the circum-oesophageal nerve connectives (PI. 4, Fig. 19, 



G. Ml). 



