Common Crayfish . 1 1 1 



segments in the insect, a very close, even if not complete, corresjiondence 

 exists between the arrangements of the ventral ganglionic chain in 

 Insecta, Araclinida, and Crustacea. 



In this Table the series of ganglia as observable in the developing 

 and in the adult forms respectively of Astacus^ Scorpio^ and Sphinx, 

 have been placed side by side in six columns, the maximum number 

 of divisions in which is seventeen, the typical number, according 

 to the view here adopted, of the post-oral segments in Arthropoda. 

 Each actually existing ganglion is placed in the particular division 

 of the scale of seventeen which is regarded as its homological 

 relation to the complete series furnished by the developing Astacus. 

 Thus a glance shows both where coalescence has taken place and 

 where ganglia have failed to be developed. 



In a second Table the ganglionic series of an Amphipodous, of an 

 Isopodous Crustacean, and of an Orthopterous insect, have been similarly 

 arranged in parallel columns, the Amphipodous Gammarus making the 

 transition from the Decapodous Astacus to the Isopodous Oniscus easy, 

 and the Oniscus in its turn appi'oximating perhaps more closely than 

 most or all other Crustaceans to the insects ^. The anatomical points 



" The order Orthoptera is believed to be the earliest representative of the class 

 Insecta in geological times, see Gerstaecker, Klassen und Ordnungen des Thier-reichs, 

 Bd. v., p, 292 ; and as an order they are distinguished by the possession of a number 

 of characteristics which approximate them to the Crustacea, the earliest geological 

 representatives of the sub-kingdom Arthropoda. Amongst these may be mentioned 

 the possession of the processes figured at c in pi. vi., and known as ' cerci anales,' 

 which appear to be homologous with certain processes which Eathke has spoken 

 of in Crustacea, e. g. Apus, Branchipus, Cyclops (Morphologie, p. 115) ; the retention 

 by the second pair of maxillae of something of their typical distinctness as opposed 

 to fusion, as in other insects, into a ' labium ;' the presence of three basal joints 

 as a support to the multiarticulate antenna which thus resembles the antennule 

 of Crustacea; and, lastly, the functional peculiarity of ecdysis, which attaches even 

 to adult Ephemeridae in this order of insects, and the pedunculate position of the 

 central eyes in certain male Ephemeridae, Chloe diptcra s. L'phemera bioctilata, 

 Linn. The internal structural arrangements of the order Isopoda, irrespective of 

 those of their nervous system, present some points of interesting resemblance 

 to those of Insecta, and especially of Orthoptera. Their long non-ramified hepatic 

 coeca are essentially similar to those seen in the Orthoptera ; see above, p. 86, 

 and pi. vi. h, and description; Leydig, Lehrbuch der Histologic, pp. 362, j?33, 

 fig. 194 ; whilst in their air-breathing genera, Oniscus and Tylos, a system of 

 canals has been discovered in the opercula of their branchial plates, which has been 

 supposed to be a rudimentary tracheal apparatus. As these peculiarities do not 

 relate to the nervous system, it is of the more importance to note that a sympathetic 



