Common Crayfish. 109 



Isopoda, the post-abdominal may be represented by but a single mass. 

 Though, as already stated, we know that in the earliest periods of de- 

 velopment of the nerve system, at least in the Crayfish and Scorpion, 

 the sin<Tle ganglionic centre which innervates the first six post-oral 

 segments, consists of an equal number of pairs of ganglia, it has been 

 shown in other cases, as by Weismann in the developing Dipterous larva, 

 and Metschinow in Aphis Eosae, that the nerve centres of many typically 

 distinct segments may from the very earliest periods be connate. As, 

 indeed, the nerve system is specialized in Arthropoda at a very late 

 period as compared with that of the segmentation of the body and the 

 development of the appendages, it may seem that when it does thus 

 come into being, it must adjust itself to any secondary arrangement or 

 disposition which these organs may have assumed in the course of their 

 evolution, rather than to any typical relations of distinctness or of one- 

 ness which they may have manifested in earlier periods. In cases such 

 as those of the larvae of Muscidae, as described by Weismann, where the 

 nerve centres of the entire series of post-maxillary segments are, if no 

 earlier and rapidly transitory period of distinctness has been overlooked, 

 connate from their first appearance, the number of nerves given off from 

 the connate mass may serve to indicate its really compound nature. But 

 though in the larvae specified, as many as eleven pairs of nerves are given 

 oft' from the single mass corresponding to eleven post-maxillary ganglia 

 of such larvae as Gorethra plumicornis or Phalaena neustria, it is well 

 to note that in the pei'fect insect, even this indication of the typical 

 plurality of nerve ganglia is wanting, and that the ventral cord of the 

 adult fly, exclusively of its first sub-oesophageal mass which supplies the 

 modified jaws, gives off" only four pairs of nerves, and one terminal 

 azygos abdominal cord. 



As in the Myriapoda the jaws are supplied from a single g-ang- 

 lion, just as in the other three classes of this sub-kingdom, we 

 may say that in all Arthropoda, the first three post-oral segments 

 upon which the mandibles and maxillae are developed, are inner- 

 vated, at least in the adult state, by a single ganglion. In Crustacea 

 and Myriapoda, this, which may be spoken of as the manducatory 

 ganglion, is always fused with more or fewer of the succeeding 

 ganglia accordingly as more or fewer of the thoracic appendages 

 are converted into auxiliary jaws. In the fourteen-footed Iledrioph- 

 thalmata therefore, where only the anterior pair of thoracic limbs 

 is converted into an accessory oral organ, we should say that the 



