434 F. W. GAMBLE AND J. H. ASHWORTH. 



very opaque, and are enclosed in a thick chorion, whicli seen 

 by oblique light appears to be perforated by minute radiat- 

 ing pores. The yolk is a light cinnamon colour. Segmenta- 

 tion is almost equal, and the embryo gradually elongates, and 

 when eighteen to twenty-four hours old acquires a belt of 

 cilia in front of which two eye-spots appear, and a broad 

 band of cilia also appears on the ventral surface. The first 

 pair of setee appears on the third day, and the mouth is by 

 that time distinct. The larvae hatch on the third day, and 

 swim freely for a day or two. The notopodial setee are first 

 formed, appearing from above downwards ; the neuropodial 

 setae appear at the end of the third or beginning of the 

 fourth day. The larvas then secrete a gelatinous tube, sink 

 to the bottom of the vessel, and creep about there. There 

 they lived for more than three weeks, and by the end of that 

 time possessed eleven to twelve setigerous segments.'" 



The post-larvee of A, marina taken at Plymouth by 

 Garstang, and described by Benham {' Journ. Marine Biol. 

 Assoc.,^ vol. iii, No. 1, 1893, p. 48), also inhabited gelatinous 

 tubes which they had secreted, and which closely invested 

 their bodies. This covering, however, did not prevent them 

 swimming actively in an eel-like manner, generally near 

 the surface of the water (Garstang). 



Small specimens of A. marina when taken from the 

 sand are beautifully transparent for some time, but when 

 placed in sea water the skin gradually becomes translucent 

 and finally opaque. Littoral forms are generally reddish or 

 yellowish red in colour, while those from the Laminarian zone 

 are much darker in colour, sometimes indeed almost black. 

 On the Lancashire coast Laminarian specimens attain a 

 length of 400 mm. 



3. External Characters. 



Although the chief external features of the Arenicolidas 

 have been carefully studied by a number of zoologists, yet 

 there is still some doubt as to the meaning of certain of 



