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DEVELOPMENT OF LARVAL NEPHRIDIA. 225 If not fed. after the third dav thev gradually dimmish in siz»
ordinary sea-water. If not fed, after thé third day they gradually dimfoish in siz» and- finally break up at the'end of a few weeks, having made no further progress in devèlopment. .S~M~SS~ As the adult worms are by po means very abundant at Naples, and thé number of eggs obtained from a single worm is never very great, I experienced considérable difliculty in obtaining sufficiènt material. On this account I adopted the môst economical method of studying the material I obtained. 1 found the combiiled celloidin-parafhn method of sectioning very useful, as by the use of this raethod very little material is lost in the process ot embedding, and it is possible to section the minutest larvée one at a time if necessary. It also has this advantage, that very small sections can be traced in a rapid manner consecutively on the slide, by means of the celloidin. On this account 1 did not adopt thé usual plan of dissolvïng it out, but allowed it to remain on the slide as a means of readily finding the sections. Material was fixed either in Flemming's strong solution, or Hermann's. For staining, hasmacalcium and otlier hsematoxylin mixtures are most useful. For larvée to be studied whole, dilute paracarmine, followed by slightly acid alcohol, gives very satisfactory results.,
While the larva of Polygordius is found in thé tow abundantly during onlythe months of February, March, and April, it is possible to rear the larva from the egg throughout ail the spring and summer months. After the middle of September I had some difticulty in obtaining the adults, so I have been unable to détermine whether the sexual products are developed all the year round. The normal discharge of thèse products seems to be restricted to the spring and, possibly, the summer months.The free-swimming larvœ are found only during the spring and summer months along the eastern coast of North America and. in the North Sea, while in the Mediterranean they are found in February, March, and April, as already mentioned. During the month of May, 1900, 1 obtained a number of Polygordius larvae from the "tow" at Woods Holl, Massachusetts, which 1 had the opportunity of comparing with the larvae found at*Naples; from these they can hardly be distinguished, and in all essential featm-es, as I have already mentioned, they are the same. I have considered it best in the foregoing work not to mention the various stages by age, as development is affected by température to such an extent that there is some uncertainty in designating stages by this^fuean's. The' progress made in development in warm weather is twice as great as in the cold months of the early spring. It is possible that under natural conditions developmeut may be still more rapid for this reason time is a somewhat incertain manner of designating them. Cowlks(3) states that he found the larva of P. appendieulatus go through metamorphosis in the course of a night under favourable conditions. He says, It surprised me tofind that fully-developed swimming larvée, takén at night; and measuring slightly less than 1 millim. in length, had metàmorphosed by the ilext morning. They were at this time 2 millims. in length, showing an increase of 1 millim. in length (p. 125).