228 ROBERT PAYNE BIGELOW ON 



SUMMARY AND CONCLUSIONS. 



Cassiopea and Polyclonia are genera of rhizostomatous medusae peculiarly modified in 

 adaptation to a sedentary mode of life in shallow water among the mangroves bordering 

 tropical seas. A comparison of specimens of Polyclonia frondosa Ag. with Cassiopea 

 xamachana shows that these two forms are specifically distinct, although in general 

 appearance they are very similar and they have the same geographical range and 

 habitat. 



C. xamachana is remarkable for its variability. This is especially shown in the 

 appendages to the mouth parts and in the structures at the margin of the umbrella. It 

 will be noticed that the most frequent number of rhopalia, in the twenty-seven specimens 

 examined, was sixteen, which is the typical number for the genus. But the variations are 

 hot arranged symmetrically on the two sides of this mode, for specimens having a 

 greater number of rhopalia are more than twice as many as those having less. The 

 species shows a strong tendency toward duplication of the rhopalia and associated struc- 

 tures of the umbrella ; and at the same time the symmetrical relations of the parts tend 

 to be preserved. The great majority of scyphomedusae have only eight rhopalia, and in 

 Cassiopea with its sixteen rhopalia we have a beautiful illustration of Darwin's law that 

 " A part developed in any species in an extraordinary degree or manner, in comparison 

 with the same part in allied species, tends to be highly variable." Study of the color 

 markings and measurements of the mouth parts indicates the division of the species into 

 three varieties; and it was in one of these, var. A, that the duplication of marginal organs 

 was especially prevalent. 



The color of both larvae and adults is due to a great extent to the presence in the 

 mesogloea of minute symbiotic algae. That these are plant cells was demonstrated by 

 micro-chemical tests. Their presence undoubtedly enables the medusae to live in water 

 that would be too poor in oxygen for most marine animals. 



The search for developing eggs proved unsuccessful, but scyphistoma larvae were 

 abundant, and it was found that they were multiplying rapidly by budding. 



The bud arises as an evagination of the body wall of the scyphistoma. There is no 

 evidence of any special gemminal epithelium. The bud, when set free, differs from a 

 planula chiefly in the possession of a well-defined mesogloea and four septal muscles. The 

 septal muscles are shown to be formed as branches of the two adjacent septal muscles of 

 the parent. The mouth of the young scyphistoma is formed by a minute perforation at, 

 the former point of attachment, while the distal end of the bud becomes the stem. This 

 remarkable orientation agrees with what Goette and Clans have found in Cotylorhita, In 



