14 THE NAUTILUS. 



group. Here to-day and gone to-morrow, perhaps abundant one 

 year and not observed again for decades, even on shores where they 

 are in most years common if rightly sought at the right season, they 

 remain practically unknown to many whose collecting is done only 

 in summer. 



A good deal of evidence has accumulated to show that many of 

 the forms, chiefly Aeolidians, have a peculiar life-history, somewhat 

 as follows : Coming on the shore in early spring they breed in the 

 rock-pools or not far below low-water mark, and almost immediately 

 die. The young, growing slowly at first and escaping observation 

 by reason of their minute size and often marvelously " protective " (?) 

 coloration, work their way slowly off shore with the coming of 

 warmer weather, migrating still further out as the cold sets in, and 

 attain their growth over winter in comparatively deep water, only to 

 perform the reverse migration, breed and die the next spring. They 

 are thus annuals. This is supposed to be the case particularly with 

 Aeolidian forms, but not even for them is the theory universally 

 accepted. There are certainly grave objections to it. It has been 

 urged that neither the on-shore nor the off-shore migration has been 

 followed ; that the ycung ought not to escape observation over sum- 

 mer even though minute and inconspicuous ; that they occasionally, 

 though rarely, occur in summer adult or nearly so ; that the dredge 

 fails to reveal them of nearly adult size in winter when they should 

 occur. On the other hand it is a fact that many of the species have 

 a fairly definite season (usually early spring, more rarely late 

 autumn, and still more rarely at other times) when in most years 

 they are witli us in fair numbers and of full size, and thereafter — 

 and suddenly — thence depart and are seen no more till the next year 

 at the same season or perhaps many years later at the same season. 

 This holds good of the rocky shores in the neighborhood of Boston, 

 and I imagine few of us have ever seen there, except in spring, more 

 than very scant and scattering examples of the Aeolid type. 



The following captures, therefore, have a distinct interest, even 

 though, by the fault of the writer, it is much less than it should be. 



On November 15, 1905, Owen Bryant, Esq., of Cohasset, Mass., 

 took from kelp dislodged by a storm from water of moderate (but 

 uncertain) depth more than sixty Nudibranchs of at least eleven 

 different species practically in company at that one spot. Not one 

 was very young (less than say 3-4 mm.) Not one was adult. 



