. Elements of comparative anatomy. Anatomy, Comparative. APPENDAGES OF THE AETHEOPODA. 245. § 189. Three pairs of the primitively similar ventral appendages are in the Insecta converted into mouth-organs; the same number are formed into feet. The former, arranged round the mouth, may at first have well served to seize and hold food, just as we see the maxillae of the Crab do at the present time. In such a stage as this the food is seized as well as comminuted. The first pair form the mandibles, and become parts of the mouth, in the form of a single joint. The second and third pairs are many-jo
. Elements of comparative anatomy. Anatomy, Comparative. APPENDAGES OF THE AETHEOPODA. 245. § 189. Three pairs of the primitively similar ventral appendages are in the Insecta converted into mouth-organs; the same number are formed into feet. The former, arranged round the mouth, may at first have well served to seize and hold food, just as we see the maxillae of the Crab do at the present time. In such a stage as this the food is seized as well as comminuted. The first pair form the mandibles, and become parts of the mouth, in the form of a single joint. The second and third pairs are many-jointed. But the basal joint only, or a few of those succeeding it, which are the nearest to the mouth, serve to comminute the food; these parts are correspondingly metamorphosed. They form the maxillae, and the remaining portion of the appen- dage looks like a jointed addition to it, and functions as a tactile organ (palp); in this way two organs, which work in different ways, are differentiated from one appendage. The most indifferent form of gnathite is found in the Aptera; in the Collembola they are sunk into the buccal cavity, and in the Thysanura they are but feebly developed. In the former when the parts of the mouth are in active use they can be protracted and drawn in again ; and thus the mouth is adapted both to biting and sucking, though, of course, to a very small extent. This indifferent condition of the organisation is developed along two distinct lines in the Pterygota. When the mandibles are well developed they have the form of cutting organs, which work on one another; the two pairs of maxillae also become cutting organs, and carry palps at the same time. This condition is permanent in the Pseudoneuroptera, Neu- roptera, and Orthoptera, although indeed points of similarity to the more indifferent stage may be seen in the first of these; and also the second pair of maxillae begins to show signs of fusion. When these gnathites are fused in the middle line the so-c
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