1877.] 



the Structure o/Magelona. 



5G3 



spicuoiis modificatious iu regard to the position of the great nerve- 

 trunks : — 



(1) Some have the trunks situated within the muscular layers, or in a 

 central hiatus between the ventral longitudinal muscles, the transverse 

 band between the latter as well as the hypoderm being external. 



(2) The cords (as in Magelonci) are distinctly hypodermic in position, 

 the oblique muscles o£ the body-wall being attached to a transverse band 

 above them, or to the summit or sides of the area containing them. 



(3) The trunks may be embraced by the closely approximated (almost 

 connate) ventral or other longitudinal muscles which overlap the 

 nerve-area. 



(4) This group is formed by those in which the cords are separate 

 throughout, being 



{a) in the substance of the ventral longitudinal muscles, 

 or (h) below or at the edge of the same muscles and within the 

 circular coat. 



The neural canals, as far as examined, occur in about thirteen families. 



Tentacles. — These remarkable organs extend to about two inches, but 

 are capable of even greater elongation. They are composed of cuticle, 

 hypoderm, basement-tissue, circular and longitudinal muscular coats, the 

 latter having a raphe at each pole in transverse section. Each forms a 

 hollow^ contractile process furnished with a series of large cylindrical 

 papillae along the anterior border, a series of central longitudinal muscular 

 fibres giving the latter appendages a sucker-action. The afferent vessel is 

 attached to the raphe next the papillae, the efferent to the raphe at the 

 smooth border. The entire organ is reproduced with considerable 

 rapidity. 



Reproduciive Orcjans. — The ova and spermatozoa are present in each 

 sex in great abundance in the posterior region of the body, and attain 

 perfection in summer and autumn. On the sides of the body, also, 

 peculiar convoluted organs occur in processes composed of the cuticle, 

 hypoderm, and basement-tissue. 



The systematic position of Magelona, with its peculiar external form 

 and internal structure, was a source of uncertainty to Dr. George John- 

 ston, the only author who attempted its consideration in this respect. So 

 puzzled was he that he placed it (as Ilcea mimhilis) at the end of his Cata- 

 logue for the British Museum, under a family specially constituted for 

 itself (viz. Mseadae). In the Catalogue of the Fauna of St. Andrews it 

 was located between the Chaetopteridae and the Spionidoe ; but the results 

 of further investigation clearly relegate it to the latter group*. It leans, 

 indeed, wholly to the Spionidae in minute structure, and especially to such 

 forms as Prionospio and Reterosjno ; though it is true that in the marked 

 regional distinctions, and the great length of the posterior division of the 



* Proc, Roy. Soc. Ediub. 1875-7(1, vol. ix, no. 94. p, 123. 

 VOL. XXY, 2 S 



