CASSIOPEA XAMACHANA. 207 



running to ciliated pockets, such as are found in Dactylometra (Bigelow, '90) , and the 

 fibres probably spread out finally into a thin network underlying the general epithelium 

 of the subumbrella. 



The one feature in which this rhopalium differs from what is found in Pelagia is the 

 presence of the pigment spot, already mentioned, lying on the aboral side of the 

 rhopalium immediately above the centre of the mass of concretions. This area is 

 probably sensitive to light, but it only differs from the rest of the sensory epithelium in 

 that here the superficial cells are deeply colored by a yellowish brown pigment. A 

 more careful examination would undoubtedly show the histology of this structure to be 

 similar to what SchewakofF ('89) has found in Aurelia. 



Habits. 



The species of Cassiopea and the closely related genus Polyclonia find their habitat 

 usually in quiet lagoons among the mangroves along the shores of the tropical seas. The 

 mode of life of several species has been described by Brandt ('38) on the authority of 

 Mertens, L. Agassiz ('62) , Gray ('69) , A. Agassiz ('81) , Archer ('81) , Fewkes ('82) , Guppy 

 ('82) , and Agassiz and Mayer ('99) . Castiiopea xamachcma is no exception to the rule 

 either in its hnbitat or its sedentary mode of life. 



When the young medusa is set free from the strobila it is an active swimmer. It 

 gradually' becomes less active as the mouth parts acquire their adult structure, and by 

 the time the animal has reached a diameter of two centimeters it has definitely taken 

 up its abode upon the bottom. It lies there, as described in the Introduction, with the 

 oral appendages upward, and seldom changes its position unless disturbed. The con- 

 cavity of the exuml.)i-ella is an important aid in maintaining this posture against the 

 action of waves and currents. The gelatinous tissue is firm and elastic, and causes the 

 umbrella to assume its normal shape when the subumbrellar muscles are relaxed. The 

 slight suction thus produced when the medusa comes to rest on a flat surface gives it 

 such a hold that a certain amount of force is required to remove it. 



Usually, however, the water in the lagoons is very quiet, and there is more danger 

 from its stagnation than from its motion. A Cassiopea is enabled to avert this danger by 

 the slight swimming movements of the thinner marginal part of the umbrella. By 

 means of these rhythmic contractions the water is drawn in on all sides, and then is 

 driven upward and away. A healthy specimen lying undisturbed on the bottom of an 

 aquarium was observed during seven minutes to make on the average 19.7 contractions 

 of the umbrella per minute. In this way the animal is enabled to dr?iw to itself a fresh 

 supply of oxygen and of its minute food material. 



