356 University of California Publications in Zoology. [Vol.4 



ancestral theca, we find at once a basis for senility of skeletal 

 parts in Ceratium. The origin of this primal theca is as yet 

 unknown, though it may well be supposed to have been formed 

 after an as yet undiscovered (for marine species) phase of sexual 

 reproduction or spore formation in which all parts of prior an- 

 cestral thecae were abandoned. It seems probable, therefore, that 

 heavily armored or senile individuals of the various species which 

 are found in any extensive collection of marine plankton owe 

 their origin to the slow accumulation of cellulose on the older 

 thecal moiety and to the accompanying assimilative regeneration 

 which gives a correspondingly rugose or senile aspect to the more 

 recently formed part of the theca, and thus to the whole organ- 

 ism. 



It is obvious that the senility here described refers strictly 

 only to the formed skeletal part of ancestral origin and does not 

 apply to the other half of the skeleton or to the cell body. It is 

 therefore a pseudo-senility. Actual senility of the whole organ- 

 ism following upon a long-continued cessation of schizogony has 

 not been detected by me, as such, though detached individuals of 

 senile facies may, indeed, belong to that category. 



6. Bemoval of wall by solution. — As before stated, total sim- 

 ultaneous ecdysis which would at once rid the organism of its 

 impeding armor has not been observed in Ceratium. Two other 

 possible methods of removal suggest themselves : partial exuvia- 

 tion and the resolution of the wall by the enveloping plasma. No 

 evidence of the latter method has been noted beyond the fact of 

 local solution in autotomy of the horns. Obviously it would be 

 difficult to detect, except by observation during the process. 

 Furthermore, it is probable that if it occurs it would exhibit the 

 same regulatory correlation that is found in thecal formation 

 and result in a reduction of all parts of the theca to a similar 

 type or facies of wall and would, therefore, be difficult to detect. 



7. Evidence of exuviation in Ceratium. — The removal of the 

 thecal wall by exuviation or progressive shedding of thecal plates 

 has not been hitherto reported. It is, however, easily overlooked 

 and the process may well be more general than the data in hand 

 indicate. Evidence of this method of removal of a greatly thick- 

 ened ancestral wall, probably at the time of fission in this in- 



