Evolution and Symmetry in the Order of the Sea-pens. 125 



there would be greater difficulty in determining the generic and specific 

 limitations in the former than in the latter, and that there should be, also, 

 a greater discontinuity between genera and species of the former, that are 

 less variable, than there is between the forms that are more variable. 



This is exactly what we do find in the order of the sea-pens. In the- 

 higher groups with very pronounced bilateral symmetry there is seldom 

 any difficulty in assigning a specimen to its proper genus. In the family 

 Pennatulid«, for example, there are four well known genera : Pennatula, 

 Leioptilum, Acanthoptilum, and Scytalium. Pennatula stands out distinctly 

 from the others in the character and distribution of the spicules, the 

 arrangement of the spicules to form the so-called calyx teeth, in the shape 

 and texture of the leaves, and in several other characters. The only difficulty 

 in this case arises in the species F. fimhriata, in which, as previously men- 

 tioned, the arrangement of the spicules on the leaves is in some specimens 

 intermediate between Pennatula and Leioptilum. Leioptilum, with its 

 closely set leaves with thick margins, is also quite distinct, Scytalium with 

 its minute flat spicules can always and at once be separated from the other 

 genera, and Acanthoptilum with its fan- shaped arrangement of flanged spicules, 

 in the leaves and minute disc-shaped spicules in the calicos is equally^ 

 distinct. In the family Pteroeididse, also the three genera Pteroeides, Sarcop- 

 tilum, and Gyrophyllum are quite distinct. Each genus is distinguished by 

 several well-marked characters, and there are no intermediate forms. 



In the Veretillidse, on the other hand, the generic groups are very indis- 

 tinct, as is particularly well shown in the instability of most of the generic 

 names. Clavella has been merged with Lituaria, Policella with Veretillum^ 

 Sarcobelemnon, Stylobelemnon, Fusticularia, and Cavernulina, with 

 Cavernularia, and the characters which are used for the separation of the 

 four remaining genera, Lituaria, Veretilluni, Cavernularia, and Actinoptilum, 

 are admittedly very variable and unsatisfactory. Lituaria differs from. 

 Veretillum only in the presence of the capstan or double-star spicules, 

 and in some specimens of Lituaria these spicules are not very abundant, and 

 are always supplemented by flat plate-like spicules similar to the prevailing- 

 type in Veretillum. Veretillum is closely related to Cavernularia, although 

 the autozooids are usually larger in the former than in the latter, and the 

 spicules of Cavernularia are usually rod-shaped or forked. Actinoptilum is 

 separated from Cavernularia by the presence of verruca? for the autozooids, a 

 character which is very variable, and depends to a considerable extent upon 

 the method of preservation. 



Moreover, there are connecting links between the Veretillidse and the 

 related families with incipient bilateral symmetry. A few rare specimens. 



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