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



groove; two pairs, with dorsal macroaepta and ventral microsepta, adjoin the former on 

 either side ; twelve pairs, with ventral macrosepta and dorsal microsepta, adjoin the latter. 

 The small dorsal and the large ventral septal regions are therefore separated on either 

 side by microsepta. There are in all thirty macrosepta and thirty microsepta. 



The following observations seem to me to justify these statements. From the 

 dissection of individual septa, it was evident to me that the oesophagus is surrounded by 

 two kinds of septa, viz., macrosepta, which are attached along the entire length of the 

 oesophagus ; and microsepta, which end on the oral disk before it becomes raised into the 

 oral lip. In all of them the muscular fibres which rise oblkpiely are very distinct, the 

 longitudinal fibres less so. 



The only example of Sphenopus arenaceus which I was able to examine was bisected 

 longitudinally parallel to the sagittal plane, so that only the one half (PL II. fig. 10) 

 contained the oesophageal groove and the septa fastened to it. At the end of the 

 oesophageal groove three macrosepta followed one another before I liberated the first 

 microseptum by dissection, whilst the adjoining part of the other half begins with a 

 microseptum, and the macrosepta and microsepta come alternately. If we then 

 compare the transverse section through Zoanthus (PI. XIV. fig. 3), we find a similar 

 arragement of the septa in the region of the oesophageal groove, except that in Sphenopus 

 the outermost of the four macrosepta placed in a row in Zoanthus is wanting. As it falls 

 in the line through which the section has been taken in dividing the animal, it has most 

 likely been destroyed. 



At the dorsal end we first meet with a microseptum, then with a macroseptum ; after 

 which, on dissection, I found the septa arranged in the following order, two microsepta, one 

 macroseptum, one microseptum, one macroseptum, one microseptum, one macroseptum. In 

 the adjoining portion of the other half, I found one microseptum, one macroseptum, One 

 microseptum, one macroseptum, two microsepta, one macroseptum. If we compare this 

 arrangement with fig. 3 of Zoanthus, and consider the two pairs of microsepta discovered 

 by dissection to be homologous with the two lateral pairs of rnicrosepta in Zoanthus, we 

 should likewise meet with the same corresponding conditions if we assume that one of 

 the small directive septa and the adjoining macroseptum have been destroyed in making 

 the section. 



Finally, as regards the number of the septa, I determined them according to the lines 

 of insertion which shone through the oesophagus ; in this way we can settle the number of 

 the macrosepta, with which the number of microsepta corresponds, presupposing, of course, 

 that they are arranged in the same way as in Zoanthus. I found this to be the case 

 in at least half of the septa dissected. 



The reproductive organs and mesenteric filaments were cemented by mucus into a 

 badly preserved mass, and were not adapted for examination. 



