1898.] PLANKTON OF THE FAEB.OE CHANNEL. 555 



of 11*5 mm., the cavity contains an amorphous plasma, which 

 disappears in a clarifying medium. The larvae of many Teleosteans, 

 e. g., Gadus, Solea, are characterized in the vitelligerous condition 

 by an expansion of the anterior part of the dorsal marginal fin, 

 the walls of which are separated and form a sinus of varying size 

 filled by a'transparent fluid 1 . The fluid being lighter than the body 

 and yolk, enables the larva to maintain a vertical position, as 

 I have been able to note by watching larvae of Gadus luscus, in 

 which the sinus is well developed. Larvae not furnished with such 

 a sinus in the vitelligerous stage are seldom vertical in position 

 when at rest, except in the case of large vigorous forms from 

 demersal ova, in which the organs of locomotion are far advanced 

 at the time of hatching. I regard the structure noted in our 

 Scopelus larvae as homologous with the sinus of early Gadoid and 

 other larvae. It may be, as Eyder supposes, a lymph-space, having 

 nothing in common except contiguity and continuity with the 

 embryonic fin-fold, but I think its function is primarily connected 

 with equilibrium. The most remarkable feature is its persistence, in 

 Scopelus, to a comparatively advanced stage of the general develop- 

 ment. In Gadus &c. it appears after hatching and attains its 

 greatest development at about the end of the vitelligerous period or a 

 little after (as in G. morrhua, teste Eyder), but disappears, so far 

 as my experience goes, before the permanent median fins commence 

 to appear. 



[(Note added Aug. 1898.) My friend and teacher, Professor Howes, 

 has called my attention to the possibility of an homology between 

 the dorsal sinus of the young Scopelus and a peculiar pad-like 

 process at the anterior end of the dorsal marginal fin of the larva 

 of Rana alticola, described by Mr. Boulenger in his Catalogue of 

 the Batrachia. Through the kindness of the last-named observer, 

 I have been able to examine a larva of R. alticola. In both cases 

 the structures are continuous with the walls of the marginal fin, 

 but they appear, at present, to have little else in common. In 

 Rana the median pad is associated with paired organs of a similar 

 nature, and all three are solid and (teste Boulenger) glandular. In 

 Scopelus the thin-walled sinus is probably devoid of well-developed 

 glandular matter, but the material is too valuable to be submitted 

 to the arbitry of the microtome. 



Although Dr. Fowler's youngest examples of Scopelus are too 

 much injured to admit of an exact determination of the extent of 

 the sinus, it appears probable that the latter covers an area 

 sufficiently extended to include the sites of all the glandular pads 

 of R. alticola. It is possible that the sinus is an organ of extreme 

 antiquity, of which the isolated pads of Rana may be modern 

 derivatives.] 



In a specimen of about the same stage of development as that 



1 Vide Eyder, Kep. Comm. Fish. U. S. A. for 1885 (1887), p. 496, pi. i. This 

 author does not regard the sinus as part of the larval fin-fold, though its walls 

 are continuous with that of the latter. 



