INVERTE3RATA. 309 



another paired metameric series of ingrowths (endosternites) which 

 arise nearly vertically from the sterna. Each endosternite further 

 sends in a process to meet one from its fellow in the middle line, 

 a series of arches being thus formed, which cover in the thoracic 

 part of the central nervous system. 



11. Ecdysis. The whole of the exoskeleton, including tendons 

 and endophragmal system, being a cuticular product, containing no 

 cells, and being largely calcified, is incapable of growth, and hence 

 a periodic moulting or ecdysis takes place once a year, during the 

 late summer, for adult crayfish, but more frequently in young ones. 

 Not only is the whole outer shell, the tendons and endophragmal 

 system shed, but even a large part of the alimentary canal has a 

 cuticular lining, which goes also. The process is a very hard and 

 exhausting one, and leaves the crayfish for some time in a very weak 

 and defenceless condition, until such time as he has a new cuticle 

 secreted and partly thickened. During this time crayfish hide in 

 their burrows. 



12. Alimentary Canal. The mouth of the crayfish is 

 a rounded aperture ; it is not defined by distinct lips, and 

 the jaws are simply modified external appendages. A short 

 oesophagus leads vertically dorsalwards into a large 

 spheroidal sac, commonly called the stomach. As it is 

 essentially a crushing organ, it is better spoken of as the 

 gizzard or gastric mill, The whole of the oesophagus and 

 gizzard is lined by a cuticle continuous with that of the 

 outer surface. In fact, they really represent a portion of 

 the outer surface which has been tucked in during develop- 

 ment. We have seen that a stomodceum, lined by epiblast, 

 forms the first part of the alimentary canal in Vertebrates, 

 and a similar proctodceum forms the anal portion. But 

 whereas in Vertebrata these epiblast-lined portions are 

 quite insignificant in comparison with the main hypoblast- 

 lined part of the gut (mesenterori), in the crayfish they 

 together form at least nine-tenths of the length of the gut, 

 and the mesenteron is quite small. 



13. The Gizzard. The cuticle of the gizzard is mostly 

 thin, but eight areas of it are thickened and calcified : these 

 are known as the " ossicles," and specially thick projections 

 from them into the cavity of the gizzard are known as 

 " teeth." All these are cuticular structures, and have no 



