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ARACHNIDA
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ARACHNIDA . Grade 4 (of the See also:ArthrOpOda).—PANTOGNATHA, TRIPROSTHOMERA. The See also:original stock, like that of the last grade, has a gnathobase on every See also:post-oral appendage, but three prosthomeres are now See also:present, in consequence of the See also:movement of the oral See also:aperture from the third to the See also:fourth somite. The later eyes are polymeniscous, with specialized vitrellae and retinulae of a definite type See also:peculiar to this grade. =See also:CRUSTACEA, CHILOPODA, See also:HEXAPODA. According to older views the increase of the number of somites in front of the mouth would have been regarded as a See also:case of intercalation by new somite-budding of new prae-oral somites in the See also:series. We are prohibited by a See also:general See also:consideration of See also:metamerism in the Arthropoda from adopting the See also:hypothesis of intercalation of somites. However See also:strange it may seem, we have to suppose that one by one in the course of See also:long See also:historical See also:evolution somites have passed forwards and the mouth has passed backwards. In fact, we have to suppose that the actual somite which in grades 1 and 2 See also:bore the mandibles lost those mandibles, See also:developed their rami as tactile See also:organs, and came to occupy a position in front of the mouth, whilst its previous See also:jaw-bearing See also:function was taken up by the next somite in See also:order, into which the oral aperture had passed. A similar See also:history must have been slowly brought about when this second mandibulate somite in its turn became agnathous and passed in front of the mouth. The mandibular parapodia may be supposed during the successive stages of this history to have had, from the first, well-developed rami (one or two) of a palp-like See also:form, so that the See also:change required when the mouth passed away from them would merely consist in the suppression of the gnathobase. The solid palpless mandible such as we now see in some Arthropoda is, necessarily, a See also:late specialization. Moreover, it appears probable that the first somite never had its parapodia modified as jaws, but became a prosthomere with tactile appendages before parapodial jaws were developed at all, or rather pari passu with their development on the second somite. It is See also:worth while bearing in mind a second possibility as to the history of the prosthomeres, viz. that the buccal gnathobasic parapodia (the mandibles) were in each of the three grades of prosthomerism only developed after the recession of the mouth and the addition of one, of two, or of three post-oral somites to the prae-oral region had taken See also:place. In fact, we may imagine that the characteristic See also:adaptation of one or more pairs of post-oral parapodia to the purposes of the mouth as jaws did not occur until after ancestral forms with one, with two, and with three prosthomeres had come into existence. On the whole the facts seem to be against this supposition, though we need not suppose that the gnathobase was very large or the rami undeveloped in the buccal parapodia which were destined to lose their mandibular features and pass in front of the mouth. Additional information and CommentsThere are no comments yet for this article.
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