Prof. S. Lov^n on the Development of the Chitons. 41 5 



its margin separates from the foot, each lamina remaining free. 

 The eyes are seen more distinctly than before to be situated on 

 the ventral side, but they are also indistinctly visible from above 

 (fig. 2). The joints of the mantle become more distinctly sepa- 

 rated, and its margin more closely set with points. The an- 

 terior, more conical part is still rather large and covered with 

 small pointed tubercles, which however are principally on the 

 upper surface. As yet I could observe no traces of the oral aper- 

 ture, and the animal was still seen sometimes swimming and some- 

 times crawling. In the meantime, the lirst layerof the shell- valves 

 was formed on the back (fig. 4), in the shape of narrow bands with 

 irregular margins, of which, as yet, I only observed seven ; the 

 three or four anterior ones being nearly equal in length, whilst 

 the posterior diminished rapidly. At this period the cirri dis- 

 appear. Thus they are wanting in the form shown in fig. 5. 



But now a considerable change takes place. We observe that 

 the conical anterior part of the animal is no longer to be seen, 

 nor the tuft of filaments which it has hitherto borne. In place 

 of this the head, with its oral opening, is perfectly developed, 

 and above it the curved folds wliich are also found in the mature 

 animal. The eyes are situated on the sides upon distinct pro- 

 tuberances, and consist of pigment-spots and lenses. The foot 

 has somewhat increased, but has not yet attained its full size in 

 proportion to the head. No trace of the branchire is yet visible, 

 but many approximated cells may be observed in the place which 

 they are to occupy. The mantle has advanced over the head, 

 and one of the shells may already be seen in front of the eyes. 

 This advancement of the mantle is distinctly shown in fig. 6. 

 In this there are still only seven distinct shells, and in front of 

 the most anterior of these is a plane surface studded with 

 pointed tubercles; this is all that remains of the conical an- 

 terior part of the embryo. This plane surface is gradually much 

 diminished, at the same time that the eighth shell is produced 

 behind the seventh. 



If the formation of the shells be examined more closely, it 

 appears in the first place, that, with the exception of the eighth, 

 they are formed almost simultaneously at the commencement ; 

 that is to say, the anterior shells are at first of a proportionate 

 size when compared with the posterior ones, which they do not 

 afterwards retain. Thus, in fig. 4, the first is a transverse arch 

 of equal length with the second and third. But this proportion 

 has already changed in fig. 6, and it alters still more subse- 

 quently. The first does not occupy the same breadth on the 

 animal as the three following ones, and thus the oval becomes 

 more distinct, as now the posterior shells increase in breadth. 



