xin ALCYONARIA ZOOIDS 33 I 



a remarkable uniformity of structure. They have eight inter- 

 mesenteric tentacles containing a cavity continuous with the 

 coelenteron. Each of these tentacles bears at least two rows of 

 simple pinnules, and they are therefore said to be " pinnate " 

 tentacles. In some species of Xenia the tentacles may have 

 three or four rows of pinnules, which give them a much more 

 feathery appearance than is usually the case. In the great 

 majority of species a single row of from eight to fourteen pinnules 

 is found disposed laterally on each side of the tentacle. The 

 mouth is usually small and slit-like with a slight rounded gape 

 at the ventral extremity. The stomodaeum is usually very 

 short, but in Xenia and in the autozooicls of some Pennatulids 

 it is relatively much longer. It is not known how far the 

 stomodaeum is of importance in the digestion of the food. In 

 Xenia 1 it has probably some importance, as shown by its unusual 

 length and the numerous large goblet cells (mucus cells) which it 

 exhibits, associated with the fact that the mesenteric filaments are 

 relatively very small. In Alcyoniuni.&nd other Alcyonaria gland 

 cells also occur in the stomodaeum, and it is probable that they 

 secrete a fluid capable of digesting to some extent the food as it 

 passes through. The most important part of the digestion, 

 however, is performed by the six " ventral " mesenteric filaments. 

 Attention has already been drawn to the fact (p. 330) that 

 two regions of the zooids of the colonial Alcyonaria can be 

 recognised. At the oral end there is a region, which in the 

 fully expanded condition consists of a crown of eight tentacles 

 surrounding the mouth, and a body-wall free from its immediate 

 neighbours. This region is called the * anthocodia." The 

 anthocodia is continuous with a region which forms a part of 

 the common colonial mass. Some genera seem to have very 

 little power of contracting the tentacles or of withdrawing the 

 anthocodiae. The zooids of Stereosoma, of Xenia, of Umbellula, 

 and of a few other genera may be described as non-retractile. In 

 many cases, however, the tentacles can be considerably con- 

 tracted, bent over the mouth, and withdrawn into the shelter of 

 the subjacent body- wall. In such a condition the surface of 

 the colony exhibits a number of tubular, conical, or convex pro- 

 tuberances, called "verrucae," and the colony is said to be 

 partially retractile. In many genera, however, the whole of the 

 1 J. H. Ashworth, Proc. Hoy. Soc. lxiii. 1898, p. 443. 





