410 Mr. E. J. Allen. Some Points in the Ili*tolo.jii ("Aj 



to the other side, and then runs down the cord. On enterii,. 

 thoracic ganglia, the fibre becomes very broad, and tnainta 

 diameter many times that of any other fibre in the ganglionic cord. 

 A single pair of elements of this kind exists, and it is easy to trace 

 the fibres from the brain through the whole length of the ganglionic 

 cord to the 6th abdominal ganglion. In the latter ganglion the fibre 

 divides into several branches, bnt I have never obtained complete 

 staining of these. The giant-fibres of the adnlt, with which these 

 agree, are stated by Retzius* to divide in the last abdominal ganglion, 

 and send branches throngh several of the nerve roots, which leave 

 that ganglion. 



In the brain this fibre gives off a few branches to the neuro 

 (fig. 1), bnt on its course down the cord, no collateral branches ha 

 ever stained, and I believe that none exist. The giant fibres differ i 

 this respect from the other fibres of the class (A) which is now bei 

 considered. 



Of the elements whose fibres do not decussate, A(6)Br. (fi^. 1) 

 may be taken as a type. The element starts with a cell of moderate 

 size on the ventral surface of the brain, immediately anterior to tin 

 large cell of the giant fibre (A()Br). The fibre passes first forward 

 and upwards, giving off numerous branches to the anterior lobes o 

 the brain on both sides, and then backwards through the brain, an< 

 down the ganglionic cord of the same side to the last abdomina 

 ganglion. It gives off collateral branches to the neuropile of th< 

 ganglia through which it passes. 



Similar elements have stained in Thorax III and Thorax IV. 



B. Each element consists of a fibre starting from a cell in one o 

 the thoracic ganglia, and running forwards to the brain. B Thorax "\i 

 aad B Thorax VI (fig. 1) are types of elements of this kind. 



The cell of B Thorax V, lies in the lateral mass of ganglioi 

 cells. Soon after entering the neuropile the fibre gives off tin 

 branches, one running forwards and breaking up in the hinder portioi 

 of the ganglion immediately anterior, whilst the other runs back 

 wards and breaks up in that immediately posterior. The main fibn 

 turns inwards, crosses its fellow of the opposite side, and then bend 

 forwards, running close to the median ganglionic cells, until it enter) 

 the brain. In the brain the fibre continues to run forwards, giving 

 off many branches, and ends at about the level of the nauplius eye 

 The fibre has not been observed to give off collateral branches dnrinj 

 its course throngh the thorax. Precisely similar elements occur u 

 Thorax VIII, and somewhat similar ones in Thorax II. 



In B., Thorax VI (fig. 1), the cell lies in the median mass o 

 ganglion cells, and the fibre, on its course towards the brain, ru 



* Retzius, " Zur Kentnias des Nervensystems der Crustaceen," ' Biol. Untersuch.' 

 Neue Folge I, 1890. 



