CHARACTERS OF COLONIAL FORMS 973 



be perfectly sound. Many early Ordovicic nautiloids show a loss 

 of the power to coil, and so indicate the existence of degenerating 

 or phylogerontic branches at a time when the class of cephalopods 

 as a whole had not yet permanently acquired the power of coiling. 

 Gastropods with the last whorl not coiled are found throughout 

 the geologic series, even in Lower Cambric time, where coiling had 

 but just begun. Such senile branches are, of course, to be expected 

 in any developing series where wrong or too hasty experiments may 

 be made by individual genetic lines. (See illustrations in North 

 American Index Fossils.) 



Colonial Forms. These require a specially modified nomencla- 

 ture since we deal not with individuals, but with groups of indi- 

 viduals. In these we must keep separate the individual life history 

 or ontogeny and the life history of the colony, i. e., colonial on- 

 togeny {astogcny or astogenesis) . The first form considered in such 

 cases is nepionic so far as the colony is concerned. To express this 

 fact Cumings (5) has coined the terms ne plastic, neanastic, ephe- 

 hastic and gcrontastic,'" which express for the colony what the 

 Hyattian terms express for the individual. The first completed 

 individual of a colony may be dignified by a distinctive term, though 

 it cannot be considered homologous with the phyl-embryonic stage 

 of the individual. In Hydrozoa and in compound corals the first 

 completed individual is the prototheca (the sicula of graptolites, the 

 initial pipe-like corallite of the Favositid corals), and in Bryozoa it 

 is the protaxium. (Cumings-5.) 



Intracolonial Acceleration and Retardation. 



Colonial organisms may suffer differential acceleration or re* 

 tardation when certain groups of individuals develop either more 

 rapidly or more slowly than others. This leads to the formation, 

 within the same colony, of two or more types of structure, normally 

 characteristic of distinct species. In this manner we can ex- 

 plain such phenomena as the occurrence of different types of leaves 

 upon the same plant, and different groups of individuals in the same 

 colony of animals where some individuals retain ancestral charac- 

 ters, while others develop further. Among plants the tulip tree 

 (Lyriodcndron titlipifera) may be taken as an illustration. The 

 two and four-lobed types of leaf are characteristic of ancestral 

 Cretacic species. Modern trees, with normally 6-lobed leaves, also 



* From dffTv (asty), a group of dwellings. 



