THE ANTHOZOA 



tentacles, which are always hollow and pinnate, the cavities of 

 the tentacles extending into the pinnae. 



There are eight mesen- 

 teries, all of which are attached 

 to the stomodaeum, and may 

 therefore be called complete. 

 There is but one longitudinal, 

 ciliated groove in the stomo- 

 daeum, which will be called 

 the sulcus, though it is not 

 certain whether the groove 

 in the stomodaeum of the 

 Alcyonarian is homologous 

 with the sulcus of the Zoan- 

 tharian zooid. The proba- 

 bility is that it is homologous. 



The mesenteries are pro- 

 vided with well -developed 

 retractor muscles, supported 

 on folds or plaits of the 

 mesogloea, which look like 

 branched processes in trans- 

 verse section, and form the 

 so-called muscle banners. The 

 arrangement of the muscle 

 banners of the Alcyonaria is 

 characteristic. They are all 

 situated on the sulcar aspects of the mesenteries (Fig. IV. 1). 



Each mesentery is provided with a mesenterial filament ; but 

 two mesenteries, namely, the asulcar pair, are longer than the rest, 

 and have a different form of filament. It has been shown by E. 

 B. Wilson (97) that the asulcar mesenterial filaments are derived 

 from the ectoderm, the remainder from the endoderm. For the 

 structure of the asulcar and other mesenterial filaments, see Fig. 

 IV. 5 and 6. 



The only exceptions to this structure are found in the arrested 

 or modified zooids which occur in many of the colonial Alcyon- 

 aria. In these the tentacles are stunted or suppressed, and the 

 mesenteries are ill-developed, but the sulcus is unusually large, 

 and is provided with specially long cilia. Such specialised zooids 

 are distinguished as siphonozooids, and their function is to drive 

 currents of water through the complex canal systems of the colonies 

 to which they belong (see Fig. XII. 4). 



Many forms of Alcyonaria have siphonozooids in addition to 

 the ordinary zooids (sometimes called autozooids), and are there- 

 fore dimorphic ; but the character is of no systematic value, for 



FIG. III. 



1. A typical Alcyonarian zooid showing the 

 eight pinnate tentacles, t ; the two long asulcar 

 mesenteries, mi and the six shorter mesenteries, 

 m 2 . (Original.) 



2. Spicules of Alcyonium digitatum. 



