CRUSTACEA 



345 



a 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. 

 233, k.op.) at the bases of the appendages from which they take their 

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



md.g. I ij 



vlv.^ lap. 



Fig. 228. The left-hand side of the fore gut and mid gut of Astaciis, viewed 

 from within. Semidiagrammatic. bri. bristles which form part of the filtering 

 apparatus ; cd. cardiac chamber of the " stomach " ; cm. median dorsal coecum 

 of the mid gut; h.g. hind gut; l.d. opening of left liver duct; la. p. lateral, 

 finely-filtering pouch of the pyloric chamber leading to opening of liver; 

 la.tth. lateral teeth; m.' , m." , left anterior and posterior gastric muscles; mdg. 

 mid gut; me.tth. median tooth; oe. oesophagus; oss. i-oss. 6, cardiac, urocar- 

 diac, prepyloric, pyloric, pterocardiac, and zygocardiac ossicles — calcifications 

 of the cuticle of the stomach which constitute the mechanism of the gastric 

 mill: when the cardiac and pyloric ossicles are pulled in opposite directions 

 by the contraction of the muscles m.' and m." attached to them, the teeth are 

 brought together; press, chamber which presses the liquid out of the food 

 into the upper passage {u.p.) and the lateral pouches {la. p.) ; py. pyloric 

 chamber; 11. p. upper passage leading to mid gut; vlv.^ one of a pair of valves 

 which guide the faecal residue from the press into vlv."^, an enclosing valve 

 which conducts it to the hind gut; vlv.^ one of a pair of valves which guard 

 openings of liver ducts. 



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 



