DEVELOPMENT OF BOLOCERA 583 
stage and show activity before the egg-membrane spines have 
disappeared. Blastulae and early gastrulae move irregularly, 
but late gastrulae and older larvae progress with the aboral 
end in advance, rotating at the same time in the contra-solar 
direction as viewed from this end. Meantime a change of 
specific gravity has occurred and the larvae tend to remain 
on or near the bottom. Elongation of the larva has also taken 
place in the oral-aboral axis. The shape now varies according 
to contraction but is usually pyriform, the aboral end being 
the smaller. Over this end the ectodermal cells elongate, 
becoming clear at their outer extremities. They are preparing 
a cement in view of fixation. At no stage is there present 
a specially elongated tuft of cilia such as is characteristic of 
the larvae of Metridium and Sagartia and in a less degree of 
Actinia equina. 
Mesenteries.—The eight primary or Edwardsia mesen- 
teries appear, first in the neighbourhood of the mouth, as folds 
of the endoderm, each containing a thin mesogloeal sheet con- 
tinuous with the general mesogloeal layer between ectoderm 
and endoderm. - The sulco-laterals (ventro-laterals) are the 
first to develop. The remainder appear practically simultane- 
ously, but I could sometimes make out that the sulculo-laterals 
were a little ahead of the sulear directives, and the latter of 
the sulcular directives. In the figures the mesenteries are 
numbered 1, 2, 3, 4, corresponding to the above sequence. 
All the primary mesenteries have appeared prior to fixation, 
and at this stage the oral ends of the sulco-laterals are already 
edged by a down-growth of stomodoeal ectoderm for the 
mesenteric filaments, and project so far inwards as almost to 
meet one another. The developing muscle banners on all the 
mesenteries show the characteristic Edwardsia arrangement. 
Fixation occurs about twenty-five days after shedding of 
the eggs, and is at first by cement attachment, the larvae 
adhering usually to the bottom but sometimes to the sides of 
the hatching vessel. The base, at first small and pointed, 
soon becomes larger and disc-like. Shortening of the larva 
takes place till the length of the column is less than its breadth ; 
the oral surface flattens ; the mouth opens widely and elongates 
