68 Illinois State Lahoratonj of NaturaJ History. 



with the cord in diameter, and to accomodate them to the 

 diminished size of the cord between ganglia the median fiber 

 is there brought down between the divisions of the central 

 nervous matter. Unlike these structures in AUolobophora, the 

 giant fibers are in this worm prqvided with a thick and well- 

 defined connective tissue sheath (Plate IV., Fig, 19, a) which 

 isolates theui from the surrounding connective tissue. The 

 axis of each fiber is hollow, and in the living worm is filled 

 with a semifluid matter which, in the sections of hardened 

 tissue, is seen as a deeply staining granular residue, sometimes 

 forming a film on the wall of the cavity, sometimes giving 

 imperfect stellate transections, and, again, filling the whole 

 space. The walls of the axial space are well-defined, and in 

 many cross sections examined I have seen a ring of small discs 

 about it, as if the wall were made up of small longitudinally 

 disposed rods, the discs being their cross sections. Focusing on 

 sections with high powers gives an appearance of fibers passing 

 from this wall into the central space.* The fibers of the 

 connective tissue sheath of the giant fibers seem to anastomose 

 with those of the ordinary connective tissue of the nerve cord. 

 The fibers of the sheath seem to join the "rods" imme- 

 diately about the axial space. Nothing has been seen of the 

 vertical septum mentioned by Dr. Leydig as dividing the cavity 

 of the median fiber in Lumbricus, and no connection between 

 the giant fibers and the nerve cells or central nervous tissue 

 has been found. 



As to the function of the giant fibers I am disposed to 

 accept Vejdovsky's view, that they are supporting structures 

 instead of parts of the nervous apparatus proper. Whether or 

 not they can be considered homologues of the notochord of 

 vertebrates must, it seems to me, be left until more has been 

 done with the embryology of invertebrates. They probably 

 originate with the sheath and connective tissue of the cord, 

 and thus independently of the essential nervous tissues.f 



* See Dr. Leydig's note on the giant fibers of earthworms. {Die 

 riesigen Nervenrohren im Bauclimark der lUngelumrmer, Zool. 

 Anz.,\8m, p. 501.) 



t Structures whicli resemble the j^iaut libers of earthworms are 

 Itif^sent ill till' vciitial cord of Cirabanis, and are said to occur also in 



