328 hitroduction to Animal Alorphology. 



terstitial calcareous deposit, pierced by many pore 

 canals* and by processes of the dermis, strong around 

 each metamere, thin in each intermetameric space, 

 so as to allow of motion, and it is periodically shed 

 and renewed. It is often prolonged for a short way, 

 or for the whole length, into the interior of the diges- 

 tive canal, and sometimes projects into the body for 

 the attachment of muscles (endophragma, or endo- 

 thorax). Uni- or multicellular hairs or bristles may 

 form as extensions of this layer, either firmly attached 

 or loose (scales of Lepidoptera). Under this is a 

 colourless, chitinogenic, cuticular layer of hexagonal 

 epithelial cells, or of continuous protoplasm, with re- 

 gularly scattered nuclei. Beneath this is the con- 

 nective dermis. The muscles are colourless, not in 

 laminae, but metamerically divided, vvith transversely- 

 striped fibres grouped in numerous bundles. f There 

 is a pharyngeal nerve ring, with an epipharyngeal 

 brain-ganglion (absent in Pentastomidae),^ and a hy- 

 popharyngeal pair of ganglia with complex commis- 

 sures,§ from which a double ventral cord extends, 

 having in each metamere a pair of ganglia whose 

 upper surfaces are motor and the lower sensory (?). 



* Which are either fine, and full of air and water, or larger, and con- 

 taining protoplastic processes of the cuticle. In Sphseroma they are wade, 

 anastomosing, and branching. 



t Lubbock describes fifty-eight in a single somite of an insect, arranged 

 as flexors, extensors, elevators, depressors, retractors, protractors, adduc- 

 tors, abductors, and rotators. The fibres do not anastomose, and are of 

 great power — thus a flea can leap 200 times its own height. 



X Not homologous with the brain of aVertebrate ; nothing but the most 

 general comparisons can be made between the nerve centres of a Vertebrate 

 and those of an Arthropod. 



\ Complex, because the head gangUa are made up of several fused me- 

 tameric ganglia. 



