Introduction to Animal Morphology. 387 



are originally pairs, but they early fuse ; the longitudinal 

 commissures, however, remain double. From the hypopha- 

 ryngeal ganglion arise the oral nerves ; from each thoracic 

 _ganglion the branches for the legs and wings of that somite ; 

 the abdominal ganglia are usually small, except the last, 

 which sends branches to the outer reproductive organs. The 

 nerve cord may extend for the whole length of the body, as 

 in Orthoptera and Neuroptera, with 6-9 abdominal ganglia. 

 In Hymenoptera there are only two thoracic ganglia and 

 1-3-4 o^ 6 abdominal. In Coleoptera there may be also not 

 only two thoracic (the meso- and meta-thoracic being fused), 

 but the abdominal ganglia may be united into one mass (La- 

 mellicornes, Scarabaeidae, Melolonthidse, Curculionidse, or 

 may be segmented into eight (Carabidae, Lucanidae, Ceram- 

 byces). Other families show intermediate conditions. In 

 Thysanura the number of ganglia in the ventral chain is vari- 

 able — twelve in Lepisma, three in Smynthurus. In some 

 Hemiptera there is a single thoracic and a single abdominal 

 ganglion. In Lepidoptera there is a union of the second and 

 third thoracic ganglia in the adult, but they are often distinct 

 in the larva ; and of the first and second, and often third ab- 

 dominal ganglia, which are also distinct in the larva. The 

 last (fourth abdominal) ganglion is bilobate, and seems 

 double. In all cases the commissures are not only laterally 

 but antero-posteriorly double, and the ventral portion has 

 been supposed to be sensitive, the dorsal to be motor, the 

 latter passes over, and fuses only partially, or not at all, 

 with the ganglia.* The nerves arise by two roots, one from 

 each of these elements. 



* The nerve cord is kept in its place by a muscular layer springing 

 from the body wall, and passing transversely over the chain of ganglia, 

 sometimes attached to the neurilemma of the interganglionic commissures 

 (Orthoptera), or also free from it (Hymenoptera), or as a meshwork of 

 irregular fibres, interlaced and inserted into the neurilemma (Diptera). In 

 Lepidoptera the neurilemma thickens, and forms a solid cord between the 

 longitudinal commissures, and into this connective axis the muscles are 

 inserted. 



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