PARKER: LONGITUDINAL FISSION IN METRLDIUM MARGINATUM. 45 



ficial to their deep ends (Plate III. Figs. 8—14). In the material at 

 hand, then, completely distinct disks were always associated with com- 

 pletely separate oesophageal tubes and single disks bearing two mouths 

 with only partially separate tubes. 



Although the oesophageal tubes have been described as though they 

 were either entirely distinct or united only through their own substance, 

 'in two of the nine animals studied there were special membranes attach- 

 ing one tube to the other. In specimen C (Plate III. Fig. 8) the two 

 tubes were held together by a single membrane which extended from 

 their oral to their pedal ends, but which failed of connection with the 

 inner surfaces of both oral and pedal disks. In specimen B (Plate II. 

 Fig. 7) two such membranes were present and extended from the region 

 where the oesophageal tubes united almost to the oral disk. Both mem- 

 branes were closely attached to the tubes, and were deficient only next 

 the oral disk ; consequently the cavity which they enclosed was entirely 

 cut off from the gastrovascular cavity except near the oral disk, where it 

 opened freely over both membranes. Although the membranes in these 

 two specimens had the general appearance of mesenteries, they lacked 

 the characteristic muscle bands and mesenteric filaments. Membranes 

 much like these, but in one case bearing mesenteric filaments, have been 

 observed in M. fimbriatum by Torrey ('98, p. 347). 



In all specimens excepting B, F, and H, each mouth was monoglyphic ; 

 in B (Plate II. Fig. 7) and F (Plate III. Fig. 11), each animal had a 

 monoglyphic and a diglyphic mouth ; in H (Plate III. Fig. 13) one 

 mouth was monoglyphic and the other aglyphic. In those cases where 

 the oesophageal tubes were united, A and B, the siphonoglyphs were, not- 

 withstanding this union, distinct throughout their whole lengths. The 

 siphonoglyphs of any double animal apparently do not occupy random 

 places, but are usually arranged symmetrically with reference to the 

 assumed plane of division. This is easily seen in Figures 8, 10, 1 2, and 

 14 (Plate III.). Its significance in connection with fission has been 

 pointed out by Torrey ('98). 



Of the complete mesenteries the directives were always in pairs, 

 and always attached to the siphonoglyphs. Their arrangement conse- 

 quently corresponded to that of the siphonoglyphs, and hence need not 

 be described. The non-directives were also always in pairs, though not 

 infrequently one member of such a pair was incomplete. This occurred 

 in eleven cases in a total of 107, or in about 10% of the mesenteries in the 

 specimens examined. Including under the head of pairs of complete 

 mesenteries those cases in which only one member of a pair is complete, 



