XXVI INTRODUCTION. 



of lobes extending the whole length of the intestine, 

 and in the Edviophthalraa, according to Prof. M. Ed- 

 wards, it is reduced to " three pairs of biliary vessels, 

 running alongside the intestine, the whole length of the 

 bod} 7 ." There are other tubular appendages connected 

 with the pyloric portion of the stomach, which are of 

 considerable size in certain of the larger Decapoda, and 

 which, from analogy, may with some probability be con- 

 sidered as pancreatic. 



The respiration in this class is, with very few excep- 

 tions amongst the Isopoda, aquatic. In some of the 

 lower forms, it would appear that there are no special 

 organs devoted to this function, but in the higher these 

 are very varied, and in many cases of a complicated 

 character. The typical form of Crustacean respiratory 

 organs may be considered that of lamellar branchiae ; and 

 this form is found in the Decapoda, and particularly in 

 the Brachyura ; in the crab it is seen in its most complete 

 development. The branchiae are placed within a distinct 

 cavity on each side, protected above by the carapace, and 

 lying upon the upper surface of the thorax. They con- 

 sist of a series of elongated pyramidal bodies, each com- 

 posed of a vast number of plates or lamellae, which are 

 closely packed, but still admit of the free circulation of 

 the water between them. The respiratory cavity has an 

 afferent and an efferent opening, through which the water 

 is propelled by a mechanism differing in the different 

 groups. The former opening, through which the water 

 has access to the cavity, is a long lateral slit, between 

 the cephalo-thorax and the side of the thorax ; and the 

 latter is near the buccal cavity, and is covered by the 



