Text-book of comparative anatomy . actory organs on the can always distinguish in theventral chord a cephalic and a trunkportion. The former consists of theinfra-cesophageal ganglion, composedof the fused ganglia of the oral limbs,which in the embryo are often trunk portion of the ventral chordmust originally have consisted of asmany double ganglia united by longi-tudinal commissures, as there are trunksegments, but the ganglia of some ofthe last trunk segments are alwaysfused to form a terminal ganglion,generally somewhat larger in size thanthe rest. The ventral chord
Text-book of comparative anatomy . actory organs on the can always distinguish in theventral chord a cephalic and a trunkportion. The former consists of theinfra-cesophageal ganglion, composedof the fused ganglia of the oral limbs,which in the embryo are often trunk portion of the ventral chordmust originally have consisted of asmany double ganglia united by longi-tudinal commissures, as there are trunksegments, but the ganglia of some ofthe last trunk segments are alwaysfused to form a terminal ganglion,generally somewhat larger in size thanthe rest. The ventral chord is foundin this unconcentrated form in theMyriapoda, Apterygota, and many Ptery-itufn, and especially in the larvas of theHcxapoda. We find, however, withinvarious orders of the Hexapoda moreor less pronounced concentration ofthe ventral chord in a way similar tothat described in connection with theCrustacea. This concentration takes place by the fusing of pairs ofganglia; it may appear in the abdomen as well as in the thorax and. Fio. 323.—Lithobius forficulatus seenfrom the ventral side (after R. Leuckart).(i, Antenna;; l;f, maxillipedes(poison feet);sd, salivary glands ; lm, ventral chord ; q>,coxal pores. VI ANTENNATA—THE NERVOUS SYSTEM 465 generally, not always, proceeds in both frombehind forward. A junction of the fused gangliaof thorax and abdomen to form a large thoracicganglionic mass may even take place (as in theBrachyura and many Copepoda); such cases occurin the Diptcm and Rliynchota. Although thelarvae generally possess a less concentratednervous system than the imagines, so that theprogressive concentration can often be followedontogenetically in the same species, this is notalways the case, in fact the very reverse occasion-ally occurs. The interesting relation betweenthe nervous system of the larva and that of theimago will be again referred to. From the ganglia of the ventral chord of thetrunk (thorax and abdomen) arise the nerves forits integument, m
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
