32 McMURRICH 



Hertwigs there is a deep incisure, extending almost to the cen- 

 tral core of mesoglcea ; I found a similar incision, somewhat 

 more extensive indeed, in one specimen I examined, but in others 

 it was entirely wanting. Carlgren ('93) states that in the indi- 

 viduals he examined the arrangement of the lamellae differed 

 from what the Hertwigs describe in being attached at their bases 

 to an axial lamella instead of a solid core ; that is, he finds the 

 sphincter is bipinnate instead of palmate as the Hertwigs describe 

 it. In a single specimen of those I examined I found a typically 

 bipinnate sphincter a figure of which is given on PI. I. (Fig. 6). 

 It seems accordingly that we may find in individuals of Tcalia 

 crassicornis, either palmate or pinnate sphincters, the latter prob- 

 ably representing what may be considered a persistence of an 

 embryonic condition from which the palmate is derived by a 

 fusion of the basal portions of the lateral lamellae. 



As to the arrangement of the musculature of the mesenteries 

 and of the tentacles and disk I find nothing to add to the de- 

 scriptions given by Carlgren. 



It is well known that the mesenteries of U. crassicornis are 

 arranged more or less perfectly on a decamerous plan. In 

 none of the specimens I have examined, however, is the total 

 number of mesenteries an exact multiple of ten ; thus in one 

 specimen there were altogether 8i pairs of mesenteries, in an- 

 other 91, and in another 104. The variation from the typical 

 number depends on irregularities in the development of the 

 younger series of mesenteries. Thus in the three specimens 

 just referred to the first three cycles were regularly decamerous, 

 their formula being 10, 10, 20, but considerable irregularity oc- 

 curred in the succeeding cycles. Thus in the specimen with a 

 total of 8 I pairs the fourth cycle was completely developed ex- 

 cept that in a primary interspace on one side of one of the di- 

 rective pairs there were only two representatives of it instead of 

 four, the two pairs which were lacking being those nearest the 

 pair of directives. To compensate for this diminution in the 

 total number, in another primary interspace there were two ac- 

 cessory pairs of mesenteries which may be regarded as repre- 

 sentatives of a fifth cycle, and in still another interspace there 



