EEPOET ON THE ACTINIARIA. 97 



over the different radial chambers (fig. 6). There is always a distinct hole in the 

 supporting lamella through which the ectodermal epithelium of the pedal disk sends a 

 cellular mass projecting like a mushroom towards the gastric space above the surface of the 

 endodermal epithelium. This epithelial mass completely closes the opening, but consists 

 of two layers of cells, firmly pressed together ; when these part asunder, which must be 

 the case when the posterior body-end of the animal becomes distended, an open canal is 

 formed in the epithelium, through which water can penetrate into the inside of the body. 

 Similar arrangements probably exist in Edivardsia ; the aboral section of this Actinia, 

 being likewise separated from the preceding by a constriction, can be alternately dis- 

 tended and contracted, and during this process the openings might be of great service. 



Tribe II. Pak actiniae. 



Actiniaria with septa united in pairs. Septa of each pair furnished with transverse 

 muscular fibres on the sides turned from one another, and with longitudinal muscular 

 fibres on the sides turned towards one another, excepting the two pairs of directive 

 septa, which are opposite one another, and have longitudinal muscles on the sides turned 

 from one another, and transverse muscles on the sides turned towards one another. 

 Number of the septa not determined by the number six. Mouth fissure-shaped, 

 oesophagus with two oesophageal grooves and two oesophageal lappets. 



I have separated two forms from the Hexactinise because the number of their antimeres 

 does not increase in multiples of six, and I have given them the name of " Paractiniae " 

 because they resemble the Hexactinise in the most important points, and therefore 

 represent a parallel group. Above all, they are furnished with oesophageal grooves and 

 have septa arranged in pairs, of which two pairs corresponding to the oesophageal grooves 

 are placed opposite one another, have longitudinal muscles on the sides turned from one 

 another, and are therefore true directive septa. The tentacles have undergone retrograde 

 metamorphosis in both forms, which differ, however, so greatly from one another that I 

 consider them as the representatives of two different families. 



Family, Sicyonid^. 



Sessile Paractinise with tetramerous arrangement of the septa ; circular muscle meso- 

 dermal ; tentacles transformed by retrograde metamorphosis into small knob-like stumps. 



The most striking characteristic of the Sicyonidse — apart from the retrograde metamor- 

 phosis of the tentacles, which is also met with among the Hexactinise — is the tetramerous 

 arrangement of the septa. Hteckel, as we know, sought in his Generelle Morphologic 

 for soft-membraned ancestors of the fossil TetracoralHa, and believed he had found 

 one of their descendants in Cerianthus. Eecent works on the anatomy of Cenanthus 



(ZOOL. CHALL. EXP. PAET XV. — 1882.) P 13 



