THE ANTHOZOA 37 



Calices with few, not more than nine, irregular pseudosepta. Coenenchymal 

 tubules small, numerous, polygonal. Genus Thecia, M. Edw. and Haime. 

 From the Wenlock limestone. FAMILY 4. CHAETETIDAE. Corallum 

 massive, consisting of long, prismatic, closely contiguous corallites, with 

 common walls. No coenenchymal tubules. Genus Chaetetes, Fischer. 

 From the Carboniferous. The family Monticuliporidae may provisionally 

 be placed here. For a full account of the fossil so-called tabulate 

 corals the reader should consult Nicholson's works (83 and 84). 



ZOANTHARIA SECOND SUB-CLASS OF THE ANTHOZOA. 



The Zoantharian zooid is distinguished from the Alcyonarian 

 zooid by the following characters : 



The tentacles are usually simple, more rarely compound or 

 foliaceous, either only six or more than eight in number, and 

 never provided with lateral pinnules. As a rule each tentacle, 

 which is always hollow, is placed over an intermesenterial space. 

 The mesenteries vary very much in number, and in the disposition 

 of their longitudinal retractor muscles, but these never have the 

 arrangement characteristic of the Alcyonaria. Each mesentery is 

 provided with a mesenterial filament, commonly of a trefoil shape 

 in section, the median lobe richly provided with gland cells and 

 nematocysts, the two lateral lobes without these structures, but 

 richly ciliated. The median lobe is derived from the ectoderm, 

 the lateral lobes from the endoderm. There are commonly two 

 ciliated grooves in the stomodaeum, named respectively the sulcus 

 and sulculus ; when one only is present it is named the sulcus. 

 The musculature is highly developed, especially on the mesenteries, 

 and the histological differentiation of the tissues is greater than in 

 the Alcyonaria. A skeleton may be absent or present ; when 

 present it is calcareous or horny, but is never in the form of 

 spicules, as in the Alcyonaria, and is always developed on the 

 surface of a special layer of ectoderm cells, which never wander 

 into the mesogloea. 



The Zoantharia may be simple or colonial ; among colonial 

 forms dimorphism is of uncommon occurrence. 



It has been shown that in the sub-class Alcyonaria the anatomy 

 of the zooids, the individual members of which the colonies are 

 composed, is remarkably constant, and therefore the modes of 

 budding, and the architecture of the colonies resulting from those 

 different modes were selected as the primary characters of taxo- 

 nomic value. It has been possible to show, with greater or less 

 certainty, that the highly differentiated and complex members of 

 the higher groups may be derived from a common Cornularia-like 

 ancestor, and the existence of a number of intermediate forms has 

 made it possible, in the case of nearly every group, to trace the 

 probable lines of divergence from the parent stock. In the 



