168 THE VOYAGE OF H.M.S. CHALLENGER. 



Moreover, the posterior caudal segments are produced, as iu the full-grown animal, at the 

 middle of the posterior margin into distinct dorsal projections, of which that on the third 

 segment is the largest. Finally, the first pair of caudal epimera begin to assume their 

 characteristic form, jutting forth anteriorly as small dentiform projections. 



The eves (fig. 9) have acquired a consideraljle dilatation in the middle anteriorly, 

 and within this dilatation the first fiiint trace of the true cornea, with its visual elements 

 and ocular pigment, may be seen ; the extremity of the eye being stiU produced into an 

 obtuse point, containing the provisional visual apparatus described above. 



The antennae have lost theii- mobility, as in the corresponding stage of Euphausia, 

 and are so modified as nearly to agree in their structure wdth those of the adult animal, 

 the two original natatory Ijranches having respectively been transformed into the scale 

 and the fiagellum. 



Four pairs of legs have developed, and the gills may be distinctly seen along the 

 sides of the trunk beneath the carapace, increasing in size posteriorlj^ the anterior pairs 

 being still only bifurcate. 



On the tail, all the pleopoda have been fully developed and adapted for swimming. 

 Also the luminous gloliules are now all well defined. 



The telson has acquired its characteristic slender form, and the largest of the terminal 

 spines (see fig. 20) are easily recognised as representing the subapical spines of the adult 

 animal. The median part of the apex is considerably produced, and its tip is not, as 

 in the former stages, truncate, but juts out us an acute angle. The two outermost spines 

 on each side are still unchanged. In one of the following stages (fig. 21) one pair of 

 the latter has withdrawn to the dorsal face of the telson, constituting the posterior pair 

 of dorsal denticles of the adult animal. 



Post-Larval Stages (figs. 10-12, 22). — In these stages the animal can easily be 

 recognised as a young Thysanopoda tricuspidata, exhibiting, as it does, all the essential 

 characteristics distinctive of that species. It still, however, retains a peculiar remnant of 

 its larval existence. For when viewed from above (fig. 10) the eyes are found to exhibit a 

 most remarkable form, unlike that of the adult animal, being drawn out at the exterior 

 side of the cornea to amamillar projection, at the tip of which the seven lenticular facets, 

 mentioned above as occurring in the larvae, form a perfectly circular area, one of the 

 lenses occupying the centre and the other six being placed in a regular circle around 

 it (see also figs. 11, 12). Within this projection the original fascicle of crystalline cones 

 appears with great distinctness, as also the dark pigment deposited at their base. On 

 the other hand, the true cornea, though considerably expanded, is still very imperfectly 

 developed, its areolation being indistinctly marked, and the pigment having a rather 

 diff'use character. In the foUowing stages this part, however, becomes gradually more 

 fully developed, while the mamillar projection is reduced in size and finally disappears 

 altogether. 



