NEUROMUSCULAR MECHANISM IN METRIDIUM 451 



doubted, but that this layer has the great nervous importance 

 attributed to it by the Hertwdgs is by no means so certain. Con- 

 trary to the statement made by Nagel ('92), the hps of actinians 

 are sensitive to many forms of stimulation, but that they and the 

 adjacent parts of the oesophagus form an important organ of 

 nervous transmission may be fairly questioned. If a Metridium 

 is cut in two except for the hps and more or less of the oral 

 part of the oesophagus, nervous transmission is seriously inter- 

 fered with. The stimulation of one half of such an animal is 

 followed by a contraction in that half alone, though if only 

 a small bridge of the column wall is left connecting the two 

 pieces transmission between them is easily accomphshed. We, 

 therefore, are led to conclude that notw^ithstanding the fact 

 that the hps and oesophagus contain nervous elements intimately 

 associated with the adjacent parts, they are not organs of special 

 nervous transmission as maintained by the Hertwigs and cer- 

 tain others workers. 



Another organ whose neuromuscular structure is of interest 

 is the acontium. In Metridium a single actontium is attached 

 to the free edge of each mesentery not far from its lower liniit. 

 These organs, as already stated, have a mesogloeal axis which 

 in transverse section is roughly T-shaped. The muscle of the 

 acontium is in the form of a series of fibers closely applied to 

 the two sides of the stem of the T. 



We agree with Carlgren ('93, p. 94, 135) in being unable to 

 confirm the statements made by the Hertwdgs ('79-80, pp. 562- 

 565) as to the neuromuscular structure of the acontium of 

 Sagartia. In their figure the muscular layer is on the opposite 

 side of the mesogloea from that on which Carlgren finds it in 

 Sagartia, and both he and we find it in Metridium. The rather 

 diagramatic figure of a transverse section of an acontium from 

 Metridium dianthus given by Havet ('01, p. 406, pi. 5, fig. 37) 

 is so sketchy as to make its interpretation difficult. This author, 

 how^ever, claims that both sense cells and muscle fibers are pres- 

 ent in this structure. Just external to each row of muscle 

 fibers is a thin layer of punctate substance which has been 

 generally regarded as nervous (compare Hertwig, '79-80, p. 564, 



