126 



Prof. S. J. Hickson. 



of Echinoptilum without any ventral track bridge the gap between the 

 Echinoptilidse and the Veretillidse, and some specimens of Sderohelevmon 

 hurgeri can only with difficulty be separated from Veretillum. 



As regards the separation of generic groups into species, there are 

 throughout the group endless difficulties owing to the want of agreement 

 among the authorities as to a working plan for recognising the difference 

 between " species " and " variety." These difficulties are the same as those 

 that are met with in nearly all the classes both of the animal and vegetable 

 kingdoms, and do not call for special comment. Nevertheless, in the study 

 of a large collection of Pennatulacea a very great difference is observed in 

 the facility with which specimens of the higher families and of the lower 

 families can be confidently assigned to their specific groups. 



In the genus Pennatula, for example, some of the species, such as 

 P. grandis, P. fimhriata, P. vmrrayi, P. naresi, and P. pliosphorea, are quite 

 distinct, and can be recognised by several well-defined characters. In 

 Scytalium three of the six species are well defined, and of the others our 

 knowledge is not yet sufficient to enable a judgment to be given as to 

 whether they are well-defined species or not. 



Even in Virgularia, a widespread genus of which a very large number of 

 species and specimens have been examined and described, several of the 

 species can be easily recognised on careful examination. In the Veretillidae, 

 however, difficulties of arranging specimens in defined specific groups are 

 practically insuperable. When a large number of specimens are obtained 

 from one locality they are found to exhibit so much variation that overlapping 

 of several described species may be discovered, and the present-day classifica- 

 tion is extremely unsatisfactor3^ Moreover, in the genus Echinoptilum, with 

 incipient bilateral symmetry, the characters that are used to separate the six 

 species that have been described are so unsatisfactory that it seems quite 

 probable that these species will prove to be only local varieties of one widely 

 distributed but very variable species. It would take too much time and 

 space to analyse more fully the specific grouping of the order. Such an 

 analysis would involve the discussion of some exceptional cases such as that 

 afforded by the genus Pteroeides in which the species appear to overlap as in 

 the radially symmetrical forms. It is only to be taken as an expression of 

 opinion based on a long study of a very large collection of specimens of 

 sea-pens that with the evolution of a bilateral symmetry and increased power 

 of movement the specific groups tend to become better differentiated. 



