MICROSCOPICAL STUDIES. 103 



The embryos have the characteristic Polychsete form, being 

 trochophores, that is, they are rounded bodies with an encircling 

 band of cilia immediately in front of the mouth, the portion in 

 front of the band, forming the prse-oral region already referred to 

 as the precursor of the adult prostomium. 



Adult form is soon assumed, but for some time after so doing 

 the young pursue a pelagic existence, swimming freely on the surface 

 of the sea, and frequently falling victims to the snare of the tow-net. 

 Like the adults these larvae are brightly phosphorescent upon irri- 

 tation — the luminous areas being the elytra. 



- The Polychsetous Annelids, or Polychsetes, to which Polynoe 

 belongs, include nearly all the marine bristle-bearing worms. Yery 

 few (only some half-dozen rarities) are inhabitants of fresh water ; 

 whereas, on the contrary, the Oligochsetes (the few-bristled annelids), 

 extremely rare in the sea, have in fresh water and upon the land, 

 practically the entire monopoly. 



Polynoe is very closely akin to the lovely iridescent Sea-mouse 

 {Aphrodite aculeata), so often dredged in sandy estuaries. Both 

 belong to the same family, Aphroditidae, but to different tribes, 

 namely to the Polynoinse and the Hermioninse respectively. The 

 former comprises a great number of species, marked off from one 

 another chiefly by differences in the number and ornamentation of 

 the scales, and in the sculpturing of the setae. 



I might have chosen a much simpler type whereby to illustrate 

 the anatomy of the Polychaetes, but on the principle of the axiom 

 that the greater includes the less, I chose Polynoe, as in it, com- 

 plexity of anatomy reaches its highest development and if we become 

 familiar with its general features, we need not be afraid of being 

 puzzled to understand the modifications seen in other forms. 



The present species, P. propinqua, is dominant in the Channel 

 Isles between tide-marks, but in the Irish Sea I found it extremely 

 rare, its place being taken by the stouter P. imbricata. Both these 

 are prowling robbers, without home or sojourning place, but many 

 species take up uninvited lodging with some of the larger tube- 

 forming worms, or with Echinoclerms. Thus two species live with 

 the strange Ghcetopterus in his great parchment tube, one with the 

 giant Eunice sanguinea, one with Terebella, and another in the 

 ambulacral groove of the starfish Astropecten, while a tiny one lies 

 sometimes among the tube-feet of Echinus, and yet another, one of the 

 most beautiful, finds safe dwelling among the spines of the purple heart 

 urchin Spatangus. Such are termed commensals or messmates, but 

 the mutual relations subsisting between the host and the guest are 

 obscure and little understood. It may be that the annelid frequents 

 the host for protection only, sallying out in quest of food when 



