. Quarterly journal of microscopical science. a knob-like or rod-like thickening of the cuticle isfitted as refractive medium. In Diplopoda (Julus, etc.) the results of embryologicalstudy point to a composition of the front part of the headexactly similar to that which we find in Onychophora. Theyare deuterognathous. The Arachuida present the first stage of progress. Hereembryology shows that there are two prosthomeres (fig. 4), 548 E. KAY LANKESTEK. and that the gnatliobases of the cheire which act as the firstpair of hemiguaths, are carried by the third somite. TheArachnida are therefore tri
. Quarterly journal of microscopical science. a knob-like or rod-like thickening of the cuticle isfitted as refractive medium. In Diplopoda (Julus, etc.) the results of embryologicalstudy point to a composition of the front part of the headexactly similar to that which we find in Onychophora. Theyare deuterognathous. The Arachuida present the first stage of progress. Hereembryology shows that there are two prosthomeres (fig. 4), 548 E. KAY LANKESTEK. and that the gnatliobases of the cheire which act as the firstpair of hemiguaths, are carried by the third somite. TheArachnida are therefore tritoguathous. The two prostho-raeres are indicated by their coelouiic cavities in the embryo(I and II, fig. 4), and by two neuromeres^ the protocerebrumand the deuterocerebrum. The appendages of the firstprosthomere are not present as tentacles, as in Peripatus andDiplopods, but are possibly represented by the eyes or possiblyaltogether aborted. The appendages of the second prostho-mere are the well-kuoAvu cheliceras of the Arachnids, rarely,. Fig. i.—Diagram of the head and adjacent region of an in the adult condition, though cmbrjologically theappendaL:es of somite II and the somite itsell are, as Jiere drawn,not actually in front of the mouth. E, lateral eye ; Ch, chelicera;m, mouth; P, piotocerebrum ; I>, deuterocerebrum ; I, II, III, IV,ccelom of the tirst, second, third, and fourth somites. (AfterGoodrich.) if ever, aiitenniform, btit modified as Ietroverts or clasp-knife fangs in spiders. The Crustacea (fig. 5) and the Hexapoda (fig. 6) agree inhaving three somites in front of the mouth, and it is probable,though not ascertained, that the Chilopoda (Scolopendra, etc.)are in the same case. The three prosthomeres or pra^oralsomites of Crustacea due to the sinking back of the mouthone somite farther than in Arachnida are not clearly indicatedby coelomic cavities in the embryo, but their existence isclearly established by the development and posi
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