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



these aside, we can count in l)oth cases twenty-eight almost equally distinct lines, which 

 appear to be grouped in pairs. 



Contrasted with the pedal disk the wall is very thick, as it measures 2-3 mm. in 

 transverse section, and by reason of its cartilaginous hardness forms at the same time a 

 most powerful protection for the parts covered by it. The surface is smooth and only 

 traversed here and there by furrows, which may, however, be absent in the living 

 animal ; the upper margin ends in pointed knobs which project like battlements above 

 the enclosed oral disk. The number of the knobs appears to be constant, as it amounted 

 to twenty-seven in both the larger and the smaller specimen examined, though they 

 differed in size. The larger and smaller knobs are placed irregularly, so that sometimes 

 both kinds alternate, sometimes several knobs of the same size lie beside one another. 



The oral disk, which springs from the wall at the base of the knobs, is as thin as 

 tissue paper and correspondingly transparent. Numerous (probably fifty-four) white 

 radial streaks denote the insertions of the underlying septa. 



The tentacles, like the oral disk, are very thin-walled and delicate, and are l"5-2"5 

 cm. long. The base is of medium breadth ; they then diminish rapidly in size, and run out 

 into a long fine point, through which even pressure cannot expel the contents of the 

 tentacles, thus showing the absence of the terminal opening common to many Actiniae. 

 They are placed in two alternating rows of twenty-seven tentacles each. The outer 

 tentacles spring immediately on the inside of the twenty-seven knobs of the wall, which 

 may therefore be regarded as clasp-like thickenings of their basal sections ; the inner 

 tentacles alternate with the outer, and are placed so close to them that their bases are 

 partially inserted into the interspaces between the outer tentacles. 



The oral disk is covered by a thin ectodermal slightly pleated layer of radial mus- 

 cular fibres, which extend as longitudinal fibres into the tentacles ; in many places it had 

 fallen away along with the epithelium lying above it. There were stiU fewer of the 

 circular endodermal muscular fibres preserved. 



The oral angle and the oesophageal grooves are very distinct in the oral fissure and 

 the oesophagus. The oesojjhageal grooves are only a little longer than the rest of the 

 oesophagus, but on the other hand they are of considerable breadth, and occupy about two- 

 fifths of the whole extent of the oesophagus. The side walls of the grooves are repeatedly 

 folded in a transverse direction. Longitudinal folds, nine in the one case, eleven in the 

 other, which begin with the same number of knobs at the margin of the oral fissure, run 

 on the two intermediate portions of the oesophageal wall. The oral margin itself projects 

 as a ridge, just as the oral disk in Sphenopus arenaceus is very much raised before it 

 passes at an acute angle into the oesophagus. 



There are, altogether, twenty-eight septa inserted into the oesophagus — veil-like, 

 extremely delicate, easily torn membranes, never pierced by septal stomata. It was 

 impossible to arrive at any decided opinion as to their structure and arrangement, both 



