116 



Prof. S, J. Hickson. 



Dutch " Siboga " Expedition in the waters of the Malay Archipelago, which 

 I have had the privilege to examine and describe.* 



In the early days of the investigations of sea-pens, when one or two 

 specimens from a single locality were all that a naturalist could examine, and 

 the range of variation within a specific group could not be determined, there 

 appeared to be a much greater discontinuity than actually occurs in nature, 

 and in the course of time a large number of generic and specific names were 

 suggested which have since been suppressed or" submerged. 



The result of the examination of the collection of sea-pens made by the 

 naturalists of the " Siboga " Expedition and of other large collections in the 

 British and Dutch Museums has led to the conclusion that many of the genera 

 and species of previous writers are but local varieties of genera and species that 

 have already been described, and further that the degree of variability is much 

 greater in those forms that exhibit radial symmetry than in those which are 

 bilaterally symmetrical. 



In order to make this point clear it is necessary to call attention to one or 

 two features of the Pennatulid colony. 



The body of a sea-pen consists of a colony of trimorphic or quadrimorphic 

 zooids, which in the most familiar genus Pennatula has the external form of a 

 feather. 



The first-formed zooid or " oozooid " becomes profoundly modified in the 

 course of its development to form the rachis and quill (or " stalk," as it is 

 called in the terminology of the Pennatulacea) of the feather and all the 

 other zooids (with a few exceptions), which are formed by gemmination from 

 the oozoid, are borne by the rachis. Of the secondary zooids one group — the 

 autozooids — exhibit the typical alcyonarian characters and alone bear 

 generative organs ; and it is the arrangement of these autozooids on the rachis 

 that gives us the external signs of the symmetry of the colony. 



In a bilaterally symmetrical genus such as Pennatula the autozooids are 

 arranged in rows on opposite sides of the rachis, leaving two broad tracks, 

 extending from the base to the apex, free from autozooids. These two 

 tracks on the rachis that do not bear autozooids are known as the dorsal and 

 ventral tracks and they can be distinguished from one another by the order 

 of succession of the autozooids in the rows. In a radially symmetrical genus 

 such as Veretillum or Cavernularia, on the other hand, the autozooids are 

 scattered quite irregularly all round the surface of the cylindrical rachis and 

 there is no trace either of a dorsal or of a ventral track free from autozooids. 



Between these two kinds of sea-pens we find a series of genera that are 

 intermediate in character as regards their symmetry. In Echinoptilum, for 

 * Hickson, S. J., ' Siboga Expeditie,' " LXXVII. Pennatulacea,'' 1916. 



