POSTLARVAL DEVELOPMENT. lOI 



lettering {a~j) throughout. In the figures mesentery a is the lower moiety 

 of the ventro-lateral pair of first-cycle mesenteries, mesentery /" is the left 

 moiety of the ventral directives, mesenteries d and e are the pair of second- 

 cycle mesenteries, while b and c are the two members of the rudimentarj'- pair 

 of third-cycle mesenteries. The incompletely represented eutoseptum to the 

 left of is a member of the primary cycle of septa, as is also the partial 

 septum to the right of /; the eutoseptum d-e belongs to the secondary 

 cycle, while the septa a-d and e-f, in figs. 54-56 are exosepta. The upper 

 margin of the section in fig. 54, is formed b}' the stomodaeal wall, while that 

 in all the subsequent figures is either the depressed disc or column wall. 

 This, upon retraction, has come to rest upon the septal edges, and adapts 

 itself to them in outline, being thrown into ridges and furrows. 



Fig. 54 represents a section of the sextant taken from the lower stomodaeal 

 region. The members of the mesenterial pair, b^ c, of fig. 11, have not j-et 

 reached this level, so that the sextant contains only the pair of incomplete 

 second-cycle mesenteries, d, e. The mesenteries a and/" are united with the 

 stomodaeal wall centrally, but their peripheral extremity has undergone 

 resorption and, therefore, is free. At this level the second-cycle mesenteries, 

 </, ^, are feebly represented, being much reduced peripherally as a result of 

 resorption. The eutoseptum d-e is broad, but the exosepta a-d and e-f on 

 each side of it are narrow and partly turned towards it. Exoseptum a-d is 

 bifurcated at its peripheral end, but before dividing is connected by synap- 

 ticula with the entosepta on each side. Within the angle of bifurcation 

 occurs a small, empty loculus. 



Plate 9, fig. 55, is from a section immediately above the level of the 

 depressed stomodaaum, so that it is bounded above b}^ the disc resting upon 

 the septal edges. At this level mesenteries d and e are larger than in the 

 former section and are without any peripheral degeneration; exoseptum e-f is, 

 united by its inner edge with eutoseptum d-e, but the bifurcated exoseptum 

 a-d is free. Within the loculus, at the angle of bifurcation of the exoseptum, 

 the rudiments of the pair of mesenteries, b, c, have appeared, but at this stage 

 there is no septal formation whatever within their entocoele. Already, there- 

 fore, it is manifest that the new pair of mesenteries does not inclose a pre- 

 viously formed exoseptum ; the two members lie close together within the 

 peripheral bifurcation of an exoseptum, a-d. 



Plate 9, fig. 56, is taken from a section at a somewhat higher level. 

 The second-cycle mesentery d is now united with the depressed disc, but 

 the other moiety {e) of the pair is still free ; in none is there any peripheral 

 resorption. Exoseptum ^-^ is once more distinct centrally from entoseptum 



