ANT HO ZO A. 735 



leading to a distinction of a dorsal or axial from a ventral or abaxial 

 aspect. The mouth is rarely circular. It is usually closed with the ex- 

 ception of one end, or of both ends which remain open, leading into well 

 marked grooves strongly ciliated known as gonidial grooves or siphono- 

 glyphes. The oesophagus is an ectodermic involution. It leads into a 

 central cavity the size and length of which vary. The part of the cavity 

 which surrounds the oesophagus is divided into a series of radial chambers, 

 completely or incompletely isolated by the mesenteries. These structures 

 are covered by endoderm supported by a mesoglaeal lamella continuous 

 with that of the body-wall, and with that of the oesophagus when they 

 are, as some always are, complete. The mesenteries which do not 

 reach the oesophagus are termed incomplete. When the mesenteries 

 are paired, the two members of every pair inclose a space which is known 

 as intra-septal, the spaces between adjacent pairs being termed inter- 

 septal 1 . Their free edges are usually bordered by thickenings or mesen- 

 terial filaments, more or less convoluted. They are ciliated, and are 

 sometimes entirely, sometimes partly, glandular, and usually contain nema- 

 tocysts. They are applied to the food when swallowed and their glands 

 secrete the digestive fluid. The tentacles are evaginations of the oral disc, 

 simple or compound, contractile, sometimes invaginable. The peristome 

 and tentacles, and sometimes the fore-part of the body, are retracted by 

 special longitudinal retractor muscles developed in the endoderm on one 

 aspect of all or some of the mesenteries. , 



Nematocysts are always present on the tentacles and in the mesen 7 

 terial filaments, in the stomodaeum, and in the Zoantharia Actiniaria also 

 on the body-wall and peristome. It appears to be rare for them to occur 

 on the surface of the colony as they do in the Alcyonarian Heliopora. 

 Their size varies, and they have usually simple threads : but in some 

 Actiniaria, and in the Madrepore Caryophyllia the thread is beset with 

 spirally arranged barbs of large size near its base. 



The colonial Anthozoa are produced by gemmation or fission from a 

 solitary form. The individuals or zooids may originate from a basal stalk 

 or stolon, or from a lamella, in which case they usually remain isolated. 

 But they often spring from the wall or side of the first zooid, and then 

 usually form a massive colony in which the individuals are united by 

 a plentiful common basis or coenosarc. They are generally connected 

 by canals variously arranged arising from their gastric cavities. The 

 latter, however, ane sometimes imperfectly separated. In some instances 

 the zooids become completely isolated. 



A skeleton is present in many simple and nearly all colonial forms. 



1 For these spaces Mr. Fowler has suggested (Q. J. M. xxv. p. 578) the terms 'entocoele' and 

 * exocoele ' respectively. 



