10 THE VOYAGE OF H.M.S. CHALLENGER. 



biradially symmetrical fundamental form is transformed into the bilaterally symmetrical. 

 I lay great stress upon this apparently unimportant consideration of the form of the 

 mouth, as it is the expression of a fundamental character in the architecture of the 

 body of the Actinia, which is, moreover, the standard for the configuration of the 

 oesophagus and the position of the septa. 



The oesophagus is a sac, flattened in the transverse direction, and open below and 

 above ; it is furnished with circular muscular fibres on its endodermal aspect, whilst it 

 has exceptionally longitudinal fibres on the ectodermal aspect, the one turned towards the 

 lumen of the tube. Its walls are solid, and only two instances have been observed in which 

 they have openings leading into the radial chambers. In the typical Actinias the lower 

 end of the tube is produced into two long lappets, which fall in the sagittal axis and 

 consequently under the two corners of the mouth, or, what is the same thing, where the 

 two wider sides of the tube meet each other. The inner side of the oesophagus is covered 

 with regularly arranged longitudinal furrows, of which two, corresponding to the angles of 

 the mouth, are conspicuous by their special breadth and depth. These furrows or grooves 

 lead from the oral angles to the oesophageal lappets, on which they run up to the end ; 

 they constitute half canals, which remain open, even when the two wider sides of the 

 oesophagus are pressed firmly against one another, and then become two canals leading 

 into the stomach (PL I. figs. 2, 5). 



As the oesophageal grooves pass at the one end on to the oesophageal lappets, so they 

 are bounded at the other end by two lip-kke swellings, which enclose the oral angle : 

 these are simply strongly-developed papillae, which are also found in varying number on 

 the oral margin, and' indicate the ends of the longitudinal ridges rising between the 

 smaller longitudinal furrows of the oesopbagus. The Zoantkeae and Ilyanthidoa form an 

 exception to what has been said ; the former have only one distinct oesophageal groove, 

 whilst in the latter there are none worth mentioning. We meet here with differences, 

 which are correlated with the structure and arrangement of the septa. 



The septa are supporting plates formed of connective tissue, which are covered on both 

 sides by endodermal epithelium, bear muscular fibres on both sides, and thus become 

 very important organs for the contraction of the body. In those Actiniae, which still preserve 

 the most primitive structure of the septa, e.g., the genus Corallimorphus, we can dis- 

 tinguish only two systems of muscles ; the fibres run for the most part longitudinally on 

 the one side, transversely on the other, forming in both cases a smooth, only slightly 

 pleated layer. Considered more closely, the former spring from the pedal disk and 

 the lower parts of the wall, and converge towards the oesophagus and central parts of 

 the oral disk, whilst the latter arise from the whole length of the wall and are inserted into 

 the oral disk and the oesophagus. In the majority of Actiniae the longitudinal layer is 

 differentiated by local, specially rich development of muscular fibres and repeated 

 pleating into a special more or less sharply-defined muscle, the retractor, which projects 



