cxx Introduction. 



Crustacea, the so-called antennary glands of Decapoda, the ' cement- 

 glands' of Cirrlpedia, and the poison-glands oiArgidus, though homo- 

 logous i^robably with each other, not being homologous with salivary 

 organs. The stomach consists very commonly of two portions, the 

 anterior one of which corresponds in function and position to the 

 gizzard of Insecta, whilst the posterior receives the ducts of what when 

 multi-ramified is spoken of as a ' liver ;' what when simply coecal is 

 spoken of as 'hepatic coeca;' and, what is sometimes considered, as 

 in Argidus, to be simply 'lateral diverticula,^ such as those often ob- 

 servable in Arachnida. The intestine, except in certain Cladocera, 

 takes a straight course to the anus. This orifice is usually situated 

 on the ventral aspect of the terminal segment, or the median 

 appendage of the terminal segment of the post-abdomen, except in 

 the Cladocera and Copepoda. An azygos coecal sac may open into 

 the digestive tract dorsally on the line of demarcation between the 

 pyloric portion of the stomach and the duodenum ; and three other 

 coeca may similarly open on the dorsal surface of the duodenum at 

 one or other part of its course. These latter coeca have been spoken 

 of as urinary organs ; they are, however, as being by no means con- 

 stantly present, probably homologous rather with the multiple 

 hepatic organs of the Scorpion than with its renal organs, or with 

 those of the other air-breathing classes. The heart is ordinarily 

 present, and is either elongated and vasiform, as in the sessile-eyed 

 and many of the lower Crustacea, or short and polygonal in the 

 Decapoda, where it gives origin to several arterial trunks ; or short 

 and semiglobular, as in Copepoda, where no arteries exist. In 

 every case blood finds its way into the interior of the organ through 

 venous inlets, which are dilated as in other Arthropoda, by the 

 recoil of the elastic alae cordis. 



Respiration is sometimes efiected by the general surface of the 

 body, specialized branchial organs are however usually present, and 

 take the shape either of tree-like or of leaf-like hollow outgrowths 

 attached either to segments or to appendages, or to both. As 

 there are no cilia in Arthropoda, the aerating surfaces have fresh 

 supplies of water brought into relation with them either by the 

 movements of appendages specially modified for the purpose, or by 

 the movements of ordinary locomotion, or by both combined. The 

 gill-covers of certain air-breathing Isopoda contain multi-ramified 

 cavities, which have been supposed to constitute a rudimentary 



