282 THE CRUSTACEA 



straining apparatus appear to be very constant throughout the 

 group, and are no doubt homologous with the very similar structures 

 found in other orders of Malacostraca. At its posterior end the 

 chitinous lining of the stomodaeum terminates in four tongue-like 

 lappets (/) (much elongated in Penaeus) which project freely into the 

 cavity of the mid-gut. 



In the other Penaeidea the stomodaeal armature is much more 

 complex than that just described. A large number of sclerites, 

 more or less calcined, are differentiated in the walls of both cardiac 

 and pyloric chambers. A dorsal and a ventral series can be dis- 

 tinguished, the dorsal and dorso-lateral pieces of the cardiac chamber 

 being in relation to the strong median and lateral teeth. From 

 this arrangement of the parts found in the Penaeidea, that charac- 

 teristic of the Reptantia may be easily derived, the chief differences 

 being due to the appearance of additional sclerites, and especially 

 of a series of intermediate pieces on the lateral walls of the two 

 chambers. The elements of the dorsal series are the more important, 

 forming as they do a system of levers moving the dorsal and lateral 

 teeth. 



The great majority of the Caridea diverge more widely from 

 the Penaeid type owing to the disappearance of the whole of the 

 dorsal series of sclerites and of the dorsal and lateral teeth associated 

 with them, the roof of at least the cardiac chamber remaining quite 

 membranous. Only the Atyidae and one or two others among the 

 families of Caridea hitherto examined possess certain elements of 

 the dorsal series well developed, but they are differently arranged 

 from those of the other Decapods. 



Mention must be made here of the gastroliths or " crab's eyes," 

 which are discoidal calcareous nodules in the lateral walls of the 

 cardiac division of the stomach in Crayfish (Astacus) and Lobsters 

 (Hornarus). They are periodically formed shortly before ecdysis 

 takes place, and are shed into the cavity of the stomach, to be 

 broken up and dissolved, apparently providing some of the material 

 necessary for the calcification of the new integument. No similar 

 structures are definitely known to occur in any other group of 

 Decapods. 



The mid-gut varies very much in length in different Decapoda, 

 but exact observations have been made only on a few types. In 

 the Crayfish (Astacus) it is exceedingly short, so that the dorsal 

 lappet which terminates the cuticular lining of the stomodaeum 

 extends through it into the beginning of the proctodaeum. In 

 most, if not all, Brachyura it is also very short. In the Lobster 

 (Homarus), however, it occupies five-sixths of the post-gastric part 

 of the alimentary canal. In species of AlpJieus (Caridea) the mid- 

 gut extends as far as the last somite, and in Paguristes it is longer 

 than the proctodaeum. From the upper surface of the mid-gut 



