78 Arenicolidae 



Post-larval stages have been taken off the British, French and 

 German coasts, from March to August, but the majority were found 

 either in March, April, or in the early part of May. Most post-larval 

 specimens of A. marina seem to take to a littoral habit before the 

 gills have been formed, or, at any rate, when only a few pairs of gills 

 are indicated : but the writer has two specimens, found free-swimming, 

 which bear the full complement of gills. In one of these the annula- 

 tion of the body is well marked, and the prostomium is proportion- 

 ately small ; this worm had reached the end of the post-larval stage 

 and would doubtless soon have settled down to its littoral habit. 



The following account, based on the examination of about thirty 

 post-larval specimens, some of which were examined alive, applies 

 more particularly to specimens about 4*5 to 6 mm. in length. The 

 prostomium (PI. X, Fig. 27) is large, conical or spatulate in form, 

 and overhangs the mouth. It bears dorsally, on each side of the 

 middle line, two to four eyes, one of which that first formed in 

 the larva is larger than the others. The peristomium is always 

 achaetous. Within it are the statocysts, the internal diameter of 

 which is '04 to *06 rum., from each of which the tube leading to 

 the exterior may be traced. The succeeding segment is achaetous 

 in all the specimens examined by the writer, but Profs. Ehlers and 

 Benham found in some of their specimens a minute chaeta in this 

 segment ; evidently this is a transitory condition, for the chaeta 

 soon disappears, leaving the segment achaetous, as it is invari- 

 ably in the adult. Both the peristomium and this segment are 

 generally rather smaller than the succeeding chaetiferous segments, 

 and both are subdivided, usually into two, by a shallow groove, so 

 that, as in the adult, the region between the prostomium and first 

 chaetiferous segment is formed of four rings. The nineteen segments 

 which follow are all chaetiferous, each bearing notopodial and neuro- 

 podial chaetae, which are described on pp. 43, 48. 



The tail has thirty to fifty segments, of which the anterior are 

 usually the smallest (see p. 38). 



The skin is glandular ; it contains numerous scattered cells, filled 

 with yellow granules, and mucus-forming cells, which secrete the 

 enveloping tube. The secondary annulation of the skin, which 

 corresponds with that of the adult, is seen in many of these post- 

 larval stages. For an account of the formation of the gills, see p. 55. 



The gut, which has already attained the adult form, usually 

 contains only a small amount of food-matter, in fine debris. The 

 blood-vessels and paired contractile hearts are well developed, and 



