. Elements of Comparative Anatomy. APPENDAGES OE AETHEOPODA. 239 }ttX category of appendages even on functional grounds. In the Mala- costraca tlie two pairs of antennae have no relation to locomotion, as may indeed be always seen from their form. The hinder pair (Fig. 123, aV) is ordinarily placed beside, and often is larger than, the anterior pair {cd). (Cf. also Fig. 128, «' ft".) All the other appendages are ventral in position. When the metameres begin to be formed they make their appearance following on the first pair of swimming-feet, found in tho ^ Nauplius ; one pair is distribut
. Elements of Comparative Anatomy. APPENDAGES OE AETHEOPODA. 239 }ttX category of appendages even on functional grounds. In the Mala- costraca tlie two pairs of antennae have no relation to locomotion, as may indeed be always seen from their form. The hinder pair (Fig. 123, aV) is ordinarily placed beside, and often is larger than, the anterior pair {cd). (Cf. also Fig. 128, «' ft".) All the other appendages are ventral in position. When the metameres begin to be formed they make their appearance following on the first pair of swimming-feet, found in tho ^ Nauplius ; one pair is distributed to each seg- ment. Like this swimming-foot, and like the second pair of antennas they have two ter- minal branches; these are not as a rule dif- ferentiated to an equal extent, for one branch is greatly developed and forms the principal part of the ajDpendage, while the other forms an appendage to it. It sometimes, however, has relations to the respiratory function, and is then largely developed. The appendages have very different functions, and are so metamorphosed as to adapt themselves to these functions. Such of these more anterior ventral appen- dages as lie near the mouth are converted into mouth-organs, and either form jaws "and jaws only, or foot-jaws. The relation between this arrangement and the concrescence seen in the cephalothorax, has been already referred to. A few pairs only are converted into gnathites in the Branchiopoda, and the remaining appendages, of which there is a very large number in most of the Phyllopoda, have very much the same characters as swimming-feet. The same thing happens in the Ostracoda, Copepoda and Cirri- pedia. In the last-mentioned group the posterior appendages are converted into the characteristic cirri (Fig. 119, W), The metamorphosis of the appendages is seen most completely in the Mala- costraca; let us examine more closely the arrange- ment seen in one of the Decapoda. AVe here find six pairs of appendages converted into
Size: 798px × 3133px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bo, bookcentury1800, bookdecade1870, bookpublisherlondonmacmillan
