CASSIOPEA XAMACHANA. 197 



Redundancy of mouth parts is not nearly as common as of the marginal structures. 

 Only two cases were observed. One specimen with an additional pair of oral arms in one 

 interradius had seventeen rhopalia, The other had eleven arms, with five subgenital 

 spaces and gonads, and this one had twenty-two rhopalia. On the other hand, five speci- 

 mens were found with twenty or more rhopalia, and perfectly normal mouth parts. 



It will be seen, then, that the number of rhopalia, which has been taken as the 

 principal generic character in the group, is a highly variable one. The number of 



Fig. A. Section through the stomach of a specimen with only 10 rhopalia and 5 oral arms, to show the abnormal 

 arrangement of gonadia and oesophageal canals. In the region marked a the margin of the umbrella presents a wide 

 space in which there are no rhopalia. go = gouad. For explanation of the other lettering see Explanation of Plates. 



marginal lobes in each paramere has been taken as one of the principal specific characters, 

 and this is, likewise, highly variable. The variation consists principally in the interpolation 

 of a small secondary lobe between two typical ones. Even in a regular and typical 

 specimen, such as is shown in Fig. 35, the position that would be taken by these secondary 

 lobes is indicated by small ridges on the dorsal surface. A specimen with a large number 

 of rhopalia is as likely to have the marginal lobes in each paramere arranged typically as 

 one having a smaller number. Conversely, a specimen with fifteen to seventeen parameres 

 is as likely as not to vary from the typical form. The variation may consist in the addi- 

 tion of two secondary lobes in the paramere, the addition of four lobes, or in a quite 

 irregular arrangement ; and this modification may affect all of the parameres alike or only 

 a portion of them. 



Throughout all of these modifications of the margins there is manifested a constant 

 regard, as it were, for the symmetry of the parts. It is very seldom that an additional 

 rhopalium appears as if attached fortuitously in some irregular way. Almost always either 

 it is in the midst of an entirely new paramere or else there is a distinct line of symmetry 

 running between two adjacent rhopalia that evidently correspond to an originally single 

 one. In other words, a paramere has been incompletely doubled, and the two parts are 



