P0LYCH.ETA OF THE L.M.B.C. DISTRICT. 227 



fact that where a species has (in the sarne district) one 

 nearly related form only, the one is nearly always found 

 to be littoral, the other to belong to deep water. It was 

 only very gradually as I gained in familiarity with annelid 

 life that there dawned upon me the full significance of the 

 fact that every organism occupies its own particular niche 

 in nature. It was as though a key to nature had been 

 placed in my hands. For instance, in this district 

 Sthenelais boa lives under stones between tide-marks; its 

 near neighbour S. limicola, characterised by but very 

 minute though constant differences, invariably frequents 

 deep water. Sabellaria alveolata is littoral; S. spinulosa 

 is always dredged. Polynoe castanea is only met with 

 among the spines of Spatang us purpureas, Acholoe aster 'i- 

 cola never elsewhere than in the ambulacral groove of 

 Astropecten. Nereis virens burrows in boulder clay, 

 while its congener N. fucata lives commensally with 

 Hermit-crabs (Pagurus). 



A few notes on embryology will be found under the 

 names of several species. Those on the development of 

 Arenicola marina are of the greatest interest, as they deal 

 with points hitherto undescribed. 



The following numbers and letters will be employed to 

 indicate the stations where the species were collected, 

 viz. : — 



Shore Collecting. 

 S. Southport and Formby (sand) 

 E. Egremont (sand, clay and stones) 

 N.B. New Brighton and Leasowe (sand) 

 H. Hilbre Island (sandstone) 

 L. Lavan sands, Bangor (mud and stones) 

 B. Beaumaris (mud and stones) 

 P. Puffin Island (limestone) 

 M. Port Erin, Isle of Man (Schist) 

 Mp. Peel, Isle of Man (Schist) 



