36 W. BALDWIN SPENCER. 



the ventral insertion of the oblique muscles. Certainly in 

 parts, and probably for the whole of their length, they are 

 attached to the oblique muscles by a definite mesentery 

 (fig. 15). 



These are by far the largest nerves, and run along the 

 surface in such a way as to suggest at first sight an homo- 

 logy between them and the two halves of the main ventral cord 

 in other Arthropods. There can, however, be little doubt but 

 that they are merely specially backward running nerves, the 

 ventral nerve-chain being concentrated. The two nerves are 

 fibrous in appearance, and contain what are apparently nuclei 

 flattened out and elongated in the direction of the length of 

 the nerve. Externally there is present a covering of con- 

 nective tissue, which is especially marked in young forms 

 (fig. 61, Ct.). As Leuckart first pointed out, the nerve mass 

 is to be regarded as equivalent to a fused series of paired 

 ganglia belonging to a much compressed ventral chain. In 

 its young form it shows clear indications of this in its internal 

 structure, whilst in the adult it is clearly divided into two 

 lateral halves by a slight median ventral and dorsal depres- 

 sion, continuous with broad fissures at the anterior and pos- 

 terior ends of the nerve mass (fig. 60). Each half of the mass 

 is composed, as stated by Stiles,' of an internal mass of fibres 

 surrounded by an external sheath of nerve-cells. The whole 

 is enclosed by a fibrous investment, some of the fibres of which. 

 Stiles says, are of the nature of connective tissue, whilst others 

 are nervous. I have not been able to detect the presence of 

 two such sets of fibres. The layer invests very closely the 

 nerve mass, and is undoubtedly continuous (i) with connec- 

 tive-tissue fibres ramifying within the substance of the cell 

 mass, and (ii) with a coat which surrounds the nerves as they 

 pass off, and which is especially well seen in the case of the 

 long posterior cords when sections of a young specimen are 

 cut (fig. 53). It is of course quite possible that nerve-fibres 

 are present, but the two sets cannot be distinguished in the 

 form now under consideration. 



' Loc. cit., p. 144. 



