CRUSTACEA 309 



so-called "peritrophic membrane" composed of a mucoid substance 

 secreted by certain cells of the epithelium. 



The principal excretory organs of the Crustacea are two pairs of 

 glands, known as the antennal and maxillary glands^ which open (Fig. 

 222, k.op^ at the bases of the appendages from which they take their 

 names. They are very rarely (Lophogastridae) both well developed at 

 the same stage in the same species, but one may succeed the other 

 as a functional organ in the course of the life history : the antennal 

 gland, for instance, is the larval excretory organ of the Branchiopoda, 

 but the maxillary gland is that of the adult ; and the Decapoda, whose 

 adult kidney is the antennal gland, sometimes use as larvae the 

 maxillary gland instead. The maxillary gland is the more widespread 

 as an adult organ, the antennary gland being functional in the adult 

 only in certain of the Malacostraca. In the Ostracoda and Leptostraca 



Fig. 218. The maxillary gland of Estheria. After Cannon, sac. end sac. 



both are vestigial in the adult. Each of these glands (Figs. 218-220) 

 has an end sac and a duct leading from the end sac to the exterior. 

 The end sac is always mesodermal and doubtless represents a vestige 

 of the coelom. The duct is sometimes (in the Malacostraca probably 

 always), a multicellular, mesodermal structure, and sometimes intra- 

 cellular and of ectodermal origin. At the junction of end sac and 

 duct there is often a sphincter. The antennal gland of the Decapoda 

 is usually very complicated. That of the crayfish lacks extensions of 

 the bladder which lie among the viscera in many other genera, as in 

 crabs. All the parts of the organs are excretory, and the function of 

 the sphincter of the end sac is perhaps to prevent the passage back 

 into that vesicle, which secretes ammoniacal compounds, of poisonous 

 products excreted in the duct. 



These glands are probably the remaining members of a series 

 of segmental excretory organs. Their mesodermal portions are 



