•340 ME. H. M. BERNAKD ON THE 



There seems to be no doubt that these muscles are specializations of the primitive 

 dUators and contractors of the oesophagus of the ancestral form. 



The mid-gut is provided with a layer of circular muscles, which appear not to be 

 transversely striated. They can be demonstrated the whole length of the mid-gut as far 

 as the stercoral pocket, and can be seen as nearly regular hoops round the alimentary 

 diverticula throughout the whole body. A distended diverticulum which has lost all 

 its contents, and consists simply of a hyaline membrane with thin muscular hoops, may 

 he easily mistaken, at first sight, for a large tracheal tube. 



Well-developed longitudinal muscles also occur outside the circular (PI. XXXIII. 

 fig. 10, and PI. XXX] I. fig. 19, Im). 



I have been unable to make out the exact arransrement of the muscles of the stercoral 

 pocket. Dufour (31) has given an elegant drawing of the stercoral pocket with longi- 

 tudinal and circular muscles crossing each, other at right angles. I have not been able 

 either to confirm or to revise this. 



The Muscles of the Diaphragm. — I have not been able to make out any special arrange- 

 ment in the muscles of the diaphragm. Anteriorly, the chitinous infoldings seem to be 

 «mooth and without a muscular layer. Posteriorly, however, there seems to be a thick 

 felt-work of muscles, w^hich, owing to the origin of the diaphragm as an infolding of the 

 cuticle, are probably dermal muscles. This felt-work seems to be specially thick round 

 the aperture through which the mid-gut passes, and I have little doubt that it acts like 

 a sphincter muscle to constrict the mid-gut. The pair of dorso-veutral muscles 

 ■descending just posterior to the diaphragm appear to have shifted forward (see PL XXX. 

 fig. 9). 



The portion of the diaphragm immediately under the mid-gut and above the neural 

 aperture bends suddenly forward and runs along as a support to the mid-gut, but 

 chiefly, I presume, ia order to constitute a more resistant attachment for the muscles of 

 the genital glands, just as the endosternite slopes backward for the powerful pedipalpar 

 muscles. 



The cliapliragm of Scorpio seems, like that of Galeodes, to be smootli chitiu anteriorly, but posteriorly 

 to be covered with a fine layer of muscles. The action of the two pairs of dorso- ventral muscles, the 

 one in front and the other behind, both in contact with the diaphragm, the former almost, the latter 

 quite, could hardly fail to constrict the alimentaiy canal, both being attached to the endosternite on 

 which the alimentary canal rests. The contraction of these muscles would squeeze the alimentary canal 

 between the upper edge of the aperture through the diaphragm and the endosternite. Whether this 

 action really takes place I cannot say, but I think it most probable (c/. section on the alimentary canal). 



PI. XXXIII. fig. 6 shows a jiair of muscles in the waist of a Spider, the contraction of which would 

 certainly constrict the alimentary canal. 



Histologically, the muscle-fibres do not differ from those of other Arachnids. They 

 are in all cases separated from each other by fine connective-tissue membranes (PI. XXX. 

 figs, 12). The fibres themselves are enveloped in an extremely delicate sarcolemma. 

 In cross-section the contractile striated bands radiate from a central mass of sarcoplasm, 

 in which is embedded a single row of nuclei. 



On reaching the cuticle, the contractile bands separate somewhat, so that the area of 

 attachment is greater than the sectional area of the muscle itself. 



