18 OLIGOCHAETA 



at the insertion of the septa, radial fibres pass along the ground substance and 

 between the longitudinal fibres ; these often give rise to the appearance of regular septa. 



The aquatic Oligochaeta are, with the exception of Phreoryctes and Pelodrilus, 

 characterized by the possession of a longitudinal muscular layer consisting of flat 

 flakes or lamellae imbedded in a granular substance. These fibres, or rather plates, 

 show no axial core such as occurs in the Lumbricidae and earthworms generally. 

 In Fridericia, as already mentioned, there is in addition to the lamellae and super- 

 ficial to them, a single row of fibres with an axial core. Hesse has lately shown 

 that in Fndericia aU these muscles (circular as well as longitudinal) are constructed 

 upon the Nematoid, not Hirudinean, plan. Ratzel had previously stated this of 

 the fibres of the inner longitudinal layer. 



The development of the muscles in Rhynchelmis has been worked out by Vejdovskt. 

 The mesoblastic cells which are to form the longitudinal muscles become spindle-shaped, 

 and the muscular fibre appears within them ; the original protoplasm becomes entirely 

 used up or nearly so in the formation of the fibre, while the nucleus atrophies. 



In the Lumbricidae (see Vejdovsky 9) there are two or three layers of cells which 

 are converted into the longitudinal muscular layer. The muscular fibres appear first in 

 the deepest cells, i. e. those nearest to the circular muscles ; a considerable number of 

 fibres appear in one cell. In the second and third rows of cells the muscular fibres are 

 only developed at the sides ; hence the pinnate arrangement in the adult. The cell 

 boundaries are finally lost and the cell-substance remaining over after the formation 

 of the fibres becomes the granular stroma lying between the fibres. The early stage 

 in the development of the longitudinal muscles of Lunibricus therefore represents the 

 condition which occurs in Rhynchelmis. 



The circular muscular layer has been generally put down as also a product of the 

 mesoblast. It seems, however, fairly clear from the observation both of Bebgh (3) and 

 Vejdovsky that epiblastic cells alone are concerned in its formation. On the other hand, . 

 in the regenerating tail of Lumbriculus (see Randolph 4) it is stated that the circular 

 muscles like the longitudinal are a product of the mesoblast. 



II. The Nervous System. 



The nervous system of the Oligochaeta is formed upon the same plan as that of the 

 higher Chaetopoda ; the cerebral ganglia communicate with a ventral ganglionated chain 

 by a circum oesophageal commissure— the entire cord lying in the body-cavity. 



In only one case is the primitive connexion of the cerebral nervous system with the 

 epidermis retained. This occurs in Aeolosoma ; in all the species of that genus which 

 have been microscopically examined, the cerebral ganglia, though projecting into 



