240 THE IJILATERALITY OF 



arrangement in the crown. Usually the proboscidal tentacles 

 stand in three or four transverse circles, the older ones (t l ) near- 

 est the end^and the youngest (/ 2 ) the farthest from it; but occa- 

 sionally the mouth of the proboscis expands very widely, like the 

 end of a trumpet, and at the same time decreases its length, by 

 which the alternating tentacles of the three circles are reduced to 

 one level, so as to lie in a single unbroken circle. The change of 

 place which they undergo in order to arrive at this position is 

 precisely that to which the feelers of the corona, at the base of 



the head, are subjected as they 

 successively develop in their re- 

 spective circles. This diagram 

 (fig. 138) presents an end-view 

 of three circles of tentacles, and 

 is intended to illustrate their rel- 

 ative position at the end of the 

 proboscis. The inner circle rep- 



resents the larger ones (i) nearest 



D E F 



Fig. 138. the mouth ; those ot the next 



outer circle (n) alternate with the first ; and in the third or out- 

 ermost circle they (m) alternate with those of both inner circles. 

 They appear, therefore, to be arranged in spirals, which, as the 

 dotted lines indicate, may wind either to the right or to the left; 

 but a study of their process of development discloses the fact 

 that their arrangement is simply one of alternation, and not that 

 of a true spiral. In order to bring the three alternating groups 

 into one, the tentacles of the two outer circles (IT, in) have but 

 to move in direct lines toward the centre, as the arrows indicate ; 

 or, what is more frequently the case, reversing the direction of 

 the arrows, the two inner (i, n) must move outwardly. 



Fig. 138. Diagramic representation of the relative position of the tentacles of 

 the proboscis of Tubularia, (fig. 136, p,~) and serving to illustrate the mode of 

 development of the tentacles of the corona (fig. 136, t). A, B, C, D, E, F, the 

 same as I, n, in, for which see the body of the work. Original. 



Fig. 139. The Scyphostoma of Aurelia flavidula. Per and Les. Magnified 

 8 diameters. The ends of the tentacles (C) are left out of view (see p. 67, fig. 



