Ou the Structure and Development of Argiope. 511 



sists of two layers of epiblast which are in contact except just at the 

 edge wbere in four places of the circumference they are separated by 

 the collection of cells secreting the setae. The outer layer of the fold 

 consists of fiat epithelial cells, the inner like the cells covering the third 

 segment, of coliimnar cells (figs. 34 and 35). The alimentary canal 

 extends through ali three segments, theìmesoblast lies between this canal 

 and the epiblast, in many places its walls are in contact, but at the poste- 

 rior end a small part of the body cavity is seen (fìg. 33 he) . On the 

 dorsal side the alimentary canal lies directly against the epiblast , the 

 mesoblast has not yet grown in between them (tig. 34) . This larva gra- 

 dually becomes the free swimming »iimbrella« larva by the following 

 changes: the anterior segment becomes more and more constricted off 

 from the remainder of the body, tili it is only connected by a compara- 

 tively fine stalk ; this gives it great freedom of movement. Its shape also 

 alters considerably , assuming the shape of umbrella and is covered ali 

 over with very fine cilia ; round the edge it bears a row of very large. 

 powerful cilia which until the animai begins its free swimming existence, 

 lie against the upper side of the head (fig. 29 and 30) . The four eye- 

 spots lie on the anterior end , those on the ventral side being slightly 

 more posterior and slightly farther apart than the two others. The part 

 of this segment hearing the eye-spots appears to correspond with the 

 anterior segment of Thecidium as described by Lacaze-Duthiers. The 

 folds which are formed by the second »segment« grow down over the 

 third, until the latter is almost completely enclosed. Upon one side, 

 which is, I believe, the ventral, the fold comes further than on the other 

 sides and laterally it does not come quite so far as in front or behind. 

 Thus the edge of the fold forms two bays laterally upon the sides of 

 which the setae are carried. The setae bave by this time grown to such 

 a length, that when they lie in their naturai position, pointiug towards 

 the axis ofthe larva, they completely cover in the posterior end of the 

 third »segment«. They are pointed and each bündle has from fifteen to 

 twenty setae (figs. 28 to 30) . 



That portion of the third segment which is uncovered by the fold of 

 the second, now becomes marked off by a constriction , and soon within 

 the epiblastic layer of cells, a small mass of the homogeneous support- 

 ing substance begins to appear (fig. 28 and 35 IV] . This will form the 

 stalk ofthe adult animai. Kowalevsky has figured the posterior »seg- 

 ment« as coated with cilia, I bave not been able to see these. 



The internai changes consist ofthe formation of two lateral bundles 

 of muscle fibres at the expense of the mesoblastic cells (figs. 34 and 



