286 MEAD. [Vol. XIII. 



the function of these cells in Nereis is not definitely known, 

 they are doubtless homologous with those in Amphitrite, since 

 they are the outcome of such peculiar and exactly similar cleav- 

 ages, and do not divide again, but become specialized unicellular 

 organs. In other forms we know nothing of the ultimate destiny 

 of these cells; they have never been observed to divide and 

 they vary in size, being quite small in Clymenella. 



The middle cells of the ventral (anterior) arms divide meridi- 

 onally both in Nereis and in Amphitrite. In Nereis we know 

 nothing further of their destiny, but in Amphitrite they show a 

 decided tendency to sink below the surface like the cells of the 

 dorsal arms ; and, although I have not followed their history, it 

 is certain that they correspond exactly in position to two pairs 

 of ventral unicellular mucous glands.* 



Several further divisions of the cross have been ascertained 

 in Nereis, Amphitrite, Clymenella, and Lepidonotus. As far as 

 known, the divisions are always strictly bilateral, in the same 

 direction, and preserve their tendency towards quadriradial 

 symmetry in all four quadrants ; but the pattern of the cross 

 becomes indistinguishable after a few divisions. 



Wilson says that in Nereis " there can be no doubt that the 

 cross gives rise in large part to the cerebral ganglion." It 

 would appear from its position in Amphitrite that it does con- 

 tribute to the formation of the ganglion, but I think that in 

 both cases the fate of the cross cells, excepting, of course, the 

 head-kidney and mucous glands above referred to, is doubtful. 



We do not know the particular fate of the cells which lie 

 between the arms of the cross, though in Amphitrite some of 

 those lying in the dorsal quadrant pass through the interruption 



* If the inference that these cells become the mucous glands is correct, indeed, 

 if it is not, we have here a good illustration of what may be called a quadriradial 

 symmetry in the trochophore, especially in the umbrella ; a symmetry by virtue of 

 which phenomena occurring in one place, not only repeat themselves on the 

 opposite side of the sagittal plane, but in each of the other three quadrants. This 

 tendency is traceable in the cleavage in the umbrellar region and in the distribution 

 of the purely larval organs, prototroch, mucous glands, etc. In a form like Lepi- 

 donotus, the quadriradial symmetry is apparently perfect, but in forms with un- 

 equal cleavage it appears to be gradually giving way to the strictly bilateral type 

 as illustrated by the series Amphitrite, Nereis limbata, Clymenella, and Nereis 

 Dummerillii. 



