29 



high power a transverse line can also be made out 

 bisecting eaeli of the dark bands, and is known as 

 Hensen's line. 



At the points where the muscles are attached to the 

 cuticle the hypodermis undergoes some modification, and, 

 moreover, the pigment is absent from those positions. In 

 many instances the points of attachment of the muscles 

 to the cuticle is indicated externally, for over such areas 

 the characteristic tubercles are frequently not developed 

 (fig. 60). 



The muscles of the walls of the alimentary canal 

 consist of single fibres, and their structure is referred to 

 in the account of tlie digestive system. 



In the head and thorax there are developed endo- 

 skeletal structures for the purpose of giving- a firm 

 attachment to certain of tiie muscles. The endoskeleton 

 of the head is known as the tentorium (Plate III., fig. 27), 

 and consists of a chitinous plate lying parallel with the 

 frontal plane, from which diverge two pairs of chitinous 

 arms extending respectively to the dorsal and ventral 

 integument of the head. Between the dorsal arms lie the 

 oesophagus and supra-oesophageal ganglion above it, and 

 the ventral arms embrace the infra-oesophageal ganglion 

 (Folsom 10). To the tentorium are attached a great 

 number of muscles, including the posterior or ventral 

 group of the dilatores pharyngii, and many of the muscles 

 moving the mouth-parts. In the thorax the endoskeleton 

 consists of three apodemes, one to each segment, and which 

 are termed respectively the antefurca, the medifurca and 

 the postfurca. These give attachment to many of the 

 muscles of the legs, as well as lending support to the 

 nervous system, and take the form of chitinous pillars, 

 which project upwards from the sterna and bifurcate into 

 a pair of forks or arms (Plate IV., fig. o7). The medi- 



