COMPARATIVE ANATOMY. 281 



last ; and, secondly, by the fusing of the anterior abdominal seg- 

 ments (usually only the first) with the thorax. On the other hand, 

 in a few insects {Macrolcpidoptera, Diptera, and Rhyfuhota), the 

 last (third) thoracic segment is joined with the abdomen. 



"The nervous system of the winged Insecta shows very great 

 variety in its arrangement ; the Diptera (Fig. 21, A — D) are par- 

 ticularly instructive. In no other order of insects are the extremes 

 so great, and yet connected by such numerous intermediate stages. 

 The series begins with the sub-order of the CuHcida;, Culiciformes, 

 TlpulidcB, Fimgicohe (e.g., Chironomus, A), which have very slightly 

 concentrated nervous systems. The ventral chord here consists of 

 an infra-oesophageal ganglion, three thoracic ganglia, and five to 

 six abdominal ganglia. The last thoracic ganglion is not simple, 

 but at least one of the anterior abdominal ganglia is fused with it. 

 The last and largest abdom.inal ganglion is also not simple ; it 

 consists of several (in Chironomus probably two) fused ganglia. 

 The concentration of the nervous system among the Diptera 

 begins in the families of the Empidce, Asilidce, TherecidcB, Xylo- 

 phugidcB. Bibionidce. {e.g., Empis, B.), where the two anterior tho- 

 racic ganglia become fused, so that there are only two thoracic 

 ganglia. In this respect the Diptera forms a contrast to other 

 insects with only two thoracic ganglia — e.g., many Coleoptera, 

 Lepidoptera, and Hymenoptera. In these cases it is the posterior 

 thoracic ganglion which consists of the two fused posterior ganglia. 

 Tabamcs (Fig. C) exhibits a nervous system, in which all the three 

 thoracic ganglia are fused into one thoracic ganglion mass. This 

 is the case in the families of the Syrphidce, Stratiomida, and 

 Tabanidce. The abdominal ganglia show a tendency to approach 

 each other and to fuse. Finally, the highest degree of concentra- 

 tion among the Diptera is shown in the Muscidce, CEstridce,, and 

 Pupiparce, where all the ganglia of the ventral chord, except the 

 infra-oesophageal ganglion, are fused into one large ganglionic mass 

 (Fig. 21, D, Sarcophaga). From this nerve a median nerve runs 

 towards the end of the abdomen, giving off nerves to the abdo- 

 minal segments at regular intervals. 



Turning now to the Second Volume, we come to a most inter- 

 esting chapter on the Ontogeny of Chiton /'<9///(Mollusca), Fig. 22. 

 The egg possesses little nutritive yolk. The segmentation is total 



International Journal of Microscopy and Natural Science. 



Third Series. Vol. VI. t 



