THE CRUSTACEA 15 



sists of a framework of movably articulated ossicles developed as 

 thickened and calcified portions of the lining cuticle of the 

 "stomach" or dilated part of the stomodaeum. These ossicles 

 bear teeth and are moved by a complex system of intrinsic and 

 extrinsic muscles. In the posterior division of the stomach a series 

 of pads and ridges beset with stiff hairs form a straining apparatus. 



The mesenteron, in most Crustacea, has its surface increased by 

 pouch-like or tubular outgrowths, which not only serve as glands 

 for the secretion of the digestive juices, but may also become filled 

 by the more fluid portion of the partially digested food and 

 facilitate its absorption. These outgrowths vary much in their 

 arrangement in the different groups. Most commonly there is a 

 single pair, which may be more or less ramified, and may form a 

 massive digestive gland (" hepato-pancreas" or "liver"). 



In a few parasites (Rhizocephala and the Monstrillidate among 

 Copepoda) the alimentary canal is absent or vestigial throughout 

 life. 



Circulatory System. The heart of the Crustacea is of the usual 

 Arthropodous type, lying in a more or less well-defined pericardial 

 sinus, with which it communicates by valvular openings or ostia. 

 In some of the Branchiopoda, such as Branchipus, the heart is of 

 the primitive tubular form, extending the whole length of the 

 body, and having a pair of ostia in each somite. Even within the 

 group of Branchiopoda, however, a progressive abbreviation of the 

 heart, with a diminution in the number of ostia, can be traced, 

 leading to the condition found in the Cladocera, where the heart is 

 a sub-globular sac and the ostia are reduced to one pair. Among 

 the Malacostraca, an elongated heart with numerous ostia is found 

 only in the Leptostraca and Stomatopoda. In other cases the 

 heart is generally abbreviated, and even where, as in the Amphi- 

 poda, it is long and tubular, the ostia are restricted in number. 

 From the heart, the blood passes into one or more arterial trunks, 

 which may ramify into a more or less extensive system of arterial 

 vessels, or may open at once into the general lacunar system of 

 the body (haemocoel). Sometimes, as in the Branchiopoda, even 

 the arterial trunks are absent, and the blood is discharged from the 

 anterior end of the heart directly into the lacunae of the haemocoel. 



In many Crustacea, especially those of small size (many 

 Copepoda and Ostracoda, Cirripedia), there is no heart and no 

 definite system of vessels, and the blood is simply driven hither and 

 thither by the movements of the body and of the alimentary canal. 



Certain genera of parasitic Copepoda (Lemanthropus, etc.) are 

 unique among Arthropoda in possessing a closed vascular system, 

 containing a coloured fluid, and shut off from the general cavity 

 of the body. The morphological relations of this system are still 

 obscure. 



