kofoid: mutations in ceratium. 239 



similar habitus but smaller size, is not wholly excluded. The chain of 

 two individuals of forma lineata (Lohmann, tig. Ill c) resembles the 

 heteromorpli iu the more rotund midbody and has its proportions, 

 though of smaller size. While the possibility of a complete transition 

 from the heteromorph cell (Hg. E) to C. Uneatum is certainly open, it 

 hardly seems to me that Lohmann's published data are adequate to es- 

 tablish this derivation or the conclusion that C. Uneatum of Baltic sum- 

 mer plankton owes its origin to heteromorpbic divisions of C. tripos. 



Included in Lohmann's figures of C. tripos halticum f. lata are two 

 isolated individuals (II c and d) wliich have the rotund midbody of the 

 other figures of f. lata and also of the heteromorph of his figure III b. 

 They have, however, a different facies. They are smaller than f. lata, have 

 a greater disproportion in the antapical horns, and the apical horn is short, 

 displaced to the right, and its axis is oblique to that of the midbody. I 

 have never seen this species with a long apical horn. This is Ceratium 

 ehrenhergi, described by me (1907) from the Eastern Tropical Pacific, 

 where it is widely distributed though relatively rare. It does not there 

 intergrade with any Ceratium similar to Lohmann's f. lata (= Ceratium 

 minus Gourret). 



In addition to the internal evidence in Lohmann's figures for holding 

 in abeyance his interpi'etation of these heteromorpbic scliizonts as sea- 

 sonal forms of C. tripos, there are certain other considerations which 

 weigh against this conclusion. 



(1) The changes between C. tripos halticum and the heteromorphs in 

 Lohmann's cliains and those observable in the chains found by me are 

 not of the type to be expected in seasonal variations in C. tripos and in 

 the subgenus Tripoceratium. In general, in warm seas long-horned spe- 

 cies of Ceratium, occur and in colder water the shorter-horned ones (see 

 Sclilitt, 1898, Chun, 1905, and Karsten, 1907). My own observations 

 on the plankton of the Pacific at San Diego and in the collections of the 

 " Albatross " confirm this, and also show that the same species grow 

 longer horns in warmer than in colder waters at the same or in different 

 localities. Minkiewitsch (1900) and Entz (1905) have noted a similar 

 difference in Ceratium in the Black and Adriatic seas between such 

 summer and winter forms. Changes are brought about by mere length- 

 ening of the horns. Usually all three share proportionally in the pro- 

 cess, though there are some species in which the apical horn is the one 

 most modified. Beyond this lengthening and some attendant changes in 

 the calibre and flexibility of the horns and in some cases changes in size, 

 which may or may not be a part of this adaptive process, these seasonal 



