STRUCTQRE OF THE NEPHRIDIA OF DINOPHILUS. 523 



The iioi'mal orange pigment of the worm gives a yellow 

 tone to the light passing through the preparation when 

 placed under the microscope, which rendered the finer 

 details of the structure of the nephridia more visible; the 

 outlines of the transparent solenocytes being moi'e clearly 

 defined in a yellow than in a pure white light.^ For the study 

 of the head-kidney of transparent Trochophore larvas I have 

 found the employment of a yellow glass screen most useful, 

 and in Dinophilus the yellow pigment serves much the 

 same purpose. 



Although I placed the worms in various solutions of methy- 

 lene blue and indigo-carmine I never succeeded in getting 

 the nephridia to take up these colours, as they readily do in 

 the White Sea species. While the nervous system and the 

 sensory cells of the epidermis absorb the colour by the 

 methyJene blue method of impregnation, the nephridia remain 

 entirely unstained. 



Lang has remarked on the difficulty of obtaining good pre- 

 parations of the flame-cells of Platodes with living material 

 on account of the inhibiting action of the cover-glass on the 

 flagella of the flame-cells. Much the same diflSculty is experi- 

 enced with Dinophilus; as soon as the worms are com- 

 pressed the action of the flagella in the nephridial canals is 

 at once stopped, being only resumed some time afterwards, 

 and then in only one or two of the nephridia at the most. It 

 is impossible to get a preparation in which all the nephridia 

 are to be seen at once active ; sometimes it will be the first, 

 sometimes it will be the third, but rarely two consecutive 

 nephridia are seen active, and it is thus difl&cult to determine 

 accurately the relations of one nephridium to another when 

 they have to be observed from different preparations. This 

 is especially so with regard to the terminal portion of the 

 nephridium, where it is a canal so delicate as to be almost 

 indistinguishable except under the most favourable conditions. 



^ Although blue violet light has greater resolving power than yellovr light, 

 it remains a fact nevertheless, that the solenocytes are more readily seen in a 

 yellow light. 



