198 BULLETIN OF THE 



which were identified as belonging to the genus Lumbriconereis, were col- 

 lected at low tide from this locality. They were found attached to the surface 

 of the mud in the form of gelatinous clusters glued together in spherical masses 

 of slime, which, when the flats are covered with water, wave to and fro with 

 passing currents. In size and general external appearance, they closely 

 resemble similar clusters of mollusk eggs found in the same place. The worm 

 embryos, however, have a more greenish color than that of the mollusk, and 

 can after a little practice be distinguished by the unaided &\e. 



The segmentation of the Arenicola egg, which is identical with that of Lum- 

 briconereis, has been described and figured by several naturalists, so that the 

 changes which take place in the egg up to the formation of the planula are well 

 known. The series of larval forms which are described and figured in tliis paper 

 opens with one where a segmentation of the ovum is completed, and extends to 

 a larva in which certain generic structures of Lumbriconereis are well marked.* 



In Fig. 1 we have represented the egg at that period in its development 

 when the smaller spheres, " micromeres," have jiartially grown about the larger 

 " macromeres," two of which seen in profile appear at one pole. When the 

 pole at which the macromeres lie is seen from above, so that they occupy the 

 centre of the circle of vision, four macromeres surrounded by the encroaching 

 micromeres can be easily seen. When viewed in profile, as in the figures 

 given, only two of the larger spheres appear. In an egg a little older, the form- 

 ing micromeres encroach still more on the pole at which the macromeres 

 approach the surface of the ovum, until ultimately the latter are wholly sur- 

 rounded by the smaller cells. 



The youngest of the planula series has a spherical and slightly ovate form, 

 bearing at one pole a clear projection easily distinguishable from the remaining 

 parts of the embryo. This projection may be called a cephalic prominence. 

 The whole interior of the larva is occupied by large nucleated cells, which are 

 easily seen through a transparent outer layer. On the pole opposite that capped 

 by the transparent cephalic projection which has been mentioned, there has 

 been differentiated from the outer surface a thin layer which marks the begin- 

 ning of the body of the v/orm. The cap-shaped cephalic prominence at the 

 upper pole is the first appearance of a head. The whole external surface which 

 lies between these prominences of the larval body is a broad ciliated zone, 

 which occupies the greater part of the external surface of the worm. 



The first important additions to a simple larva girt by a broad band of cilia, 

 which we have just described, is the formation, at either pole, in the clear 

 spaces which we have mentioned, of small prominent pigment spots. Five of 

 these are formed at the cephalic, and four at the caudal pole. Those which 

 appear in the cephalic prominence are three in number, placed one medially, 

 with one on each side near the pole, and one on each side near the equatorial 



* These larvae resemble closely the young Arenicola marina {piscaloruni) of Max 

 Schultze (Abhand. d. Natur. Gesell., 1856), or those of A. cristata, Stimp., described 

 by Dr. E. B. Wilson, op. cit. The larvje doubtfully referred to Lumbriconereis by 

 Claparede and Met.schnikoff (op.' cit.) are generically different from mine. 



