500 F. W. GzVMBLE AND J. H. ASHWORTH. 



those in the example, 78 mm. long, figured in Pis. 28 

 and 29. 



In conclusion it may be said that, with the exception of A. 

 Claparedii, the median fibre is constant in all species, and 

 arises from median giant-cells segmentally arranged at the 

 anterior and posterior ends of the body ; while the lateral 

 fibres are more inconstant, pursue an intricate anastomosing 

 course, and are related to the giant-cells of the middle region of 

 the body. The cells are as well, if not better, developed in 

 immature than in full-sized individuals, and in all cases exhibit 

 the characters of unipolar ganglion-cells. The exact connec- 

 tion of the giant-fibre with the cord, and especially the ter- 

 mination of its branches and of those of the more inconstant 

 lateral giant-fibres, will require further investigation, but we 

 have shown that in A. Grubii the branches accompany the 

 peripheral nerves, and are connected by short offsets with the 

 fibrous matter of the cord^ thereby increasing their likeness 

 to efferent nerve-fibres. 



12. Sense-organs. 



The otocysts are the best developed and most characteristic 

 sense-organs of the Arenicolidse. In addition to these, 

 however, there are (1) the eyes, (2) the sensitive surface of 

 the prostomium, and (3) the nuchal oi'gan or ciliated groove. 

 As stated on p. 444, we have not been able to prove that the 

 cirriform hollow processes of the tail of Arenicola cristata 

 are definitely sensory, but both these and the tubercles 

 placed just below the last few pairs of gills (which may also 

 prove to be reduced cirri) probably contain sense-ox'gans. 

 The constant and rapid movements of the notopodial bristles 

 suggest that on these a sensory ending may occur. Indeed, 

 Retzius {' Biologiska Foreningeus Forhandlingar,' Band iii, 

 Hefte 4 — 6, 1891, p. 85) has shown that in A. marina, 

 nerve-endings may be traced to the bases of these setee. 



The Otocysts. — Ehlers (1892) has given an almost ex- 

 haustive account of these interesting structures, but we have 

 gone over the ground again, since one species, Arenicola 



