218 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



is being determined by transverse section only, and tbe 

 section not being quite horizontal passes lower on the 

 one side than on the other, it may show the mesenteries 

 of the same circle incomplete on the one side and 

 complete on the other. It is of the greatest possible 

 importance to bear these points in mind, in deciding 

 the question as to whether the hexamerous arrangement 

 is retained or not, and special care should be taken to 

 distinguish the primary and secondary circles of mesen- 

 teries from each other. 



(Esophageal Grooves or Siphonoglyphs.—The instances 

 of variation found by me in Actinia equina are all in 

 the number and position of the oesophageal grooves, and 

 it is in these particulars that other investigators have 

 found most variation in other species. Out of 131 speci- 

 mens of Metridium marginatum, a common American 

 species, Parker (1897) records that 59 per cent had only 

 one siphonoglyph, a single specimen had three and the 

 remaining 41 per cent only had two. G. Y. Dixon (1888, 

 p. 120) observed that in Sagartia venusta, S. nivea, and 

 S. miniata, one or two siphonoglyphs may occur; the 

 brothers Dixon (1891, p. 19), confirmed by the observa- 

 tions of several others, show the presence of either one or 

 two in Metridium dianthus; while Haddon and t- hackle- 

 ton (1893) record that from two to seven occur in different 

 specimens of Condijlactis ramsayi. 



In the case of Sagartia spongicola, previously referred 

 to, McMurrich (1897) records variations in this respect 

 in all the seven specimens examined. He speaks of 

 the "directive" mesenteries only, but in the absence 

 of any statement to the contrary, it may be concluded 

 that each pair of directives had an " oesophageal groove ' 

 associated with it. He describes one with two pairs of 

 directives, but not arranged opposite each other, five with 



