Mr. J. D. Dana on Zoophytes. Ill 



or mud, which fact seems to require an extension of the aerating 

 surface, such as this delicate branching affords. 



The number of tentacles appears to have a relation to two di- 

 stinct series ; in one the number is six, twelve, or more, with the 

 alternate usually unequal when exceeding six (and always so 

 when over twelve) ; in the other, eight simply, and all equal. 

 The character of the tentacles is different in the two series : the 

 former (the Actinaria) having them naked, the latter (Alcyonaria) 

 having them fringed with perforate papillae. A large number of 

 species of Actinaria are characterized by twelve tentacles, and 

 have been separated to form the group Madreporacea ; but not- 

 withstanding this point of resemblance, the several genera are as 

 closely related to species having a greater number of tentacles. 



The occurrence of suctorial vesicles on the lateral surface or 

 disc is a character of only generic importance. 



Colour is seldom of much importance, even for trivial distinc- 

 tions ; yet the mode of arrangement of colours may be charac- 

 teristic of species. A mutual dependence or relation of certain 

 colours may possibly be hereafter ascertained, by which a know- 

 ledge of one will determine the others that may be possible in a 

 species ; and in such a case, the character may have a value which 

 cannot now be allowed it. 



V. Secretion of Coral. — 1. The secretion or non-secretion of 

 coral internally is at the best no more than a family distinction ; 

 and among the Alcyonaria it is only generic. This is an admitted 

 truth with regard to calcareous secretions among Mollusks ; and 

 Olivi and Blainville long since acknowledged it with reference to 

 zoophytes. 



2. The secretion of coral at base, distinct from internal secre- 

 tions, is a characteristic of much value ; it produces the structure 

 of the Gorgonia and allied zoophytes, and also of the Antipathi. 



3. The nature of coral sec?'etions sometimes affords generic 

 distinctions, and with other characters, in some instances, distin- 

 guishes the higher divisions of zoophytes. The Hydroidea, as 

 far as known, never form any but membranous or horny coralla. 

 The Astrseacea, Caryophyllacea and Madreporacea secrete only 

 calcareous coralla, excepting a few marine Actinida (Actinecta) 

 which form a cellular membranous float at base to keep them at 

 the surface. The Antipathi form only basal horny secretions, 

 and therefore have a horny axis. The Alcyonaria are more va- 

 rious in this character, the different genera having their pecu- 

 liarities : the internal secretions are always calcareous and in grains 

 or spicules, and in this last particular they differ from the calca- 

 reous of the Actinaria ; these grains are sometimes so abundant 

 as to unite into solid tubes {Tubipord). The basal secretions 

 are either horny {Gorgonina) , calcareous (Corallium), or siliceous 



