CH. X] THE BODY-WALL AND MUSCLES 203 



turned in and thickened. The internal chitinous linings of the 

 stomodaeum and proctodaeum are also directly continuous with 

 the external cuticle, just as the cells secreting them are continuous 

 with the hypodermis. 



The Hypodermis (hy) is a continuous single layer of regular 

 hexagonal flattened cells, closely underlying the cuticle throughout. 

 It serves not only for the production of the latter, but also for 

 the attachment of the body-muscles. The muscle-fibril is inserted 

 into the inner border of the hypoderm cell, which shews in the 

 region of attachment a change from a granular to a fibrillar 

 structure of the cytoplasm (fig. 88, cells above m). Here and 

 there, enlarged hypoderm cells are met with having larger nuclei 

 than usual. These receive the terminations of sensory nerve-fibres, 

 and serve to give a perception of general sensation to the body-wall, 

 as distinct from the sense-organs. By this arrangement of muscles 

 and nerves it becomes clear how, at ecdysis, the cuticle is shed 

 without any local damage to the underlying parts. 



The Haemocoele. The large open space below the hypodermis 

 is filled with blood, and is called the haemocoele (p. 162). It is 

 quite distinct from the coelome or body-cavity of Vertebrates. 

 This latter, in the Dragonfly as in all Arthropods, practically 

 disappears after embryonic development is completed. Its only 

 remnants are the pericardial cavity and the lumina of the developing 

 gonads. 



The Muscles. 



In the Dragonfly all the muscles are striated, without exception. 

 Zawarsin [203] has conclusively demonstrated the striated character 

 of the alary muscles in the Dragonfly larva. 



The muscle fibres are bound together into bundles or fasciculi, 

 with or without an enveloping sarcolemma. In the larva, the 

 muscles are all of one kind, closely resembling the striated 

 muscles of Vertebrates. In the imago, we meet with two 

 main classes of muscle, known as leg-muscle and wing-muscle 

 respectively (Weissmann). The former class includes the head 

 muscles and the segmental muscles of the abdomen. In these 

 there is only a single bundle, with a definite sarcolemma. In the 



