48 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



and was kept under observation there during August and later at Bev- 

 erly Bridge till October 3d of the same year, a period of some eight 

 weeks. During this time the animal was kept in open sea water in a 

 marked locality, not in a laboratory aquarium, which my former expe- 

 rience had taught me might be unfavorable. When first seen its pedal 

 disk measured about three centimeters in diameter. It had two com- 

 plete oral disks, one of which had a monoglyphic and the other a di- 

 glyphic mouth. The cleft between the two oral disks was a deep one, 

 and I hoped soon to witness the separation of the two individuals. 

 When about eight weeks after capture it was last seen, it had increased 

 in size so that its pedal disk was nearly seven centimeters in diameter, 

 but the cleft between the two parts had in no wise increased. The speci- 

 men then unfortunately disappeared from its locality. The fact that it 

 grew considerably shows that it was under approximately normal condi- 

 tions, and yet so far as fission was concerned the animal was essentially 

 at the same stage at the end of the eight weeks as at the beginning. Tliis 

 coincides with the experience of Torrey ('98, p. 351), who in describing 

 longitudinal fission in M. fimbriatum states that he has '' not observed 

 a single instance of full severance of individuals, though a number of 

 dividing polyps have been kept in the laboratory for nine months." 

 Thus direct evidence of actual fission is wanting. 



If now we turn to the specimens which have thus far been described, 

 it must be admitted that they can be arranged in a series passing from 

 less to more completely divided individuals. This in itself, however, 

 affords no more grounds for concluding that j\I. marginatum reproduces 

 by longitudinal fission than a similar series of partially double mammals 

 would establish longitudinal fission for these animals. To make the 

 proof conclusive the final products of the process must be found. 



As a rule, the larger specimens of ]\I. marginatum are sessile animals.^ 

 In large specimens that had undergone division the offspring ought 

 therefore to be found together. M. marginatum further shows great 

 variability in its colors and markings ; hence such natural pairs might 

 thus be distinguished from their neighbors, for the two descendants 

 would of course inherit directly the surface markings of their progenitor. 



A search was made for natural pairs at Beverly Bridge, a locality 



^ According to my own observation tlie locomotor activity of this species be- 

 comes rapidly less as the size increases. A specimen whose diameter was about 

 5 mm. moved as much as 9 cm. in twenty-four hours. A second one whose average 

 diameter was about 2 cm. never moved more than 1 cm. in a day, and a large one 

 whose diameter was about 8 cm. showed no perceptible movement in fifteen days. 



