NEUROMUSCULAR MECHANISM IN METRIDIUM 437 



producing near their pedal ends small acontia. Finally at 

 the angle of the oral disc and the column on the one hand and 

 at that of the pedal disc and column on the other hand, pairs 

 of small membranes appear which are e^ddently the beginnings 

 of pairs of mesenteries not yet united over their middle portions. 

 These incipient mesenteries sometimes show traces of filaments 

 and acontia. 



Aside from the fact that the mesenteries in Metridium are 

 almost invariably disposed in pairs and are divisible into direc- 

 tives, non-directives, and incomplete mesenteries, there is very 

 little about them that suggests the typical hexactinian. This is 

 due to the great irregularity in their arrangement, a condition 

 which has already been foreshadowed in the statement that 

 Metridium is more generally monogl>TDhic than diglyphic and 

 may even be triglj^Dhic. 



Moreover in digljT^hic specimens the mesenteries are not 

 necessarily arranged on the typical hexactinian plan. This 

 matter has been much discussed, not only for Metridium (Par- 

 ker, '97; McMurrich, '97; Torrey, '98; Parker, '99; Torrey, '02; 

 Carlgren, '04; Hahn, '05; Carlgren, '09), but also for the closely 

 allied genus Sagartia (Davenport, '03; Carlgren, '04; Torrey 

 and Mery, '04; Carlgren, '09; Davis, '09), and the outcome seems 

 to be that the irregularities in the number and arrangement of 

 the siphonoglyphs and mesenteries in these forms are due to 

 the prevalence among them of non-sexual methods of reproduc- 

 tion and, in the case of Metridium, particularly of basal frag- 

 mentation. But this question is not one of concern at the pres- 

 ent, for, whether the siphonoglyphs and mesenteries in Metri- 

 dium are regularly or irregularly arranged, the neuromuscular 

 structure of this animal follows an essentially uniform plan. 



H. THE MUSCLES 



Thirteen fairly well-defined muscles or groups of muscles 

 can be distinguished in Metridimn. Many of them were long 

 ago described and need only a word of comment, while others 

 have not been recognized until recently. 



