THE EUWAEDSIA-STAGE OF LEBETTNIA, 299 



Under a high magnification the very delicate retractor-inuscle 

 fibres on the meseuteries can be made out, enabling the two 

 pairs of directives (III., III. ; IV., IV.) to be determined. These 

 are found to be arranged about an axis ia-e) at right angles to 

 the median axis of the tentacles (y-c). I have represented the 

 cut ends of the muscular fibrils in the figure, although they 

 could not be discerned at such a low magnification as that 

 given. 



The next figure is from a section a little below the stomod^al 

 region, and, although no trace of any tentacles occurs, the 

 section follows the others iu regular sequence without any dis- 

 turbance of the axes. The larva was sectionized in one con- 

 tinuous ribbon and mounted accordingly. i\s in the previous 

 figure, the sulco-sulcular plane of the larva {a-e) is at right 

 angles to what would be the median plane of the tentacles 

 (y-e) ; in other words, the two median lateral tentacles com- 

 municate the one with the sulcar («) and the other with the 

 sulcular endocoele (e), while the two tentacles in the dorso-veatral 

 plane communicate each with a median mesenterial space (c, a). 



Another condition is disclosed by the slightly flattened larva 

 from which figs. 26-28 are taken. The series indicates that on 

 the left side the small tentacle next the large dorsal tentacle {f) 

 communicates with the sulcular endocoele (e), and the small 

 tentacle («) on the right side, next tbe ventral (6), communicates 

 with the sulcar endocoele («), so that the median axis of the 

 tentacles {f-V) is in a plane oblique to the sulco-sulcular axis 

 of the mesenteries {a-e). Here, again, the longer axis of the 

 oesophagus is in the same plane as the sulco-sulcular plane of 

 the mesenteries. 



A third larva was in the same condition as the first example 

 described — the median axis of the tentacles at right angles to 

 the axis of symmetry of the mesenteries. And, yet again, a 

 fourth example, traced later, revealed the same oblique relation- 

 ships as the second here recorded. 



In every case the longer axis of the oesophagus coincided with 

 the axis of symmetry of the pairs of mesenteries, not with that 

 of the tentacles. 



It is very evident, therefore, from these four cases, that the 

 symmetry of the two sets of organs — tentacles and mesenteries 

 — is independent in the bilateral stage of the larva. It may 



