332 Introduction to Animal Morphology. 



wards.* It has, above, the labium, and below, a lower 

 lip (in Isopods and Amphipods), or a bifid tongue 

 laterally lie the masticatory limbs and their approxi- 

 mator and divaricator muscles. The salivary glands 

 are absent or unicellular in Copepoda and Cladocera ; 

 largest in Gyge. The narrow oesophagus dilates into 

 a crop in some Entomostraca, and ends by projecting 

 into the stomach, whose anterior part is generally 

 gizzard-like, muscular, and lined by chitinous tooth- 

 processes, shed at each moult. The deepest portion 

 of the chitinous teeth is calcified in Decapoda, forming 

 a solid mass (Crab's eyes), pierced by pore-canals. 

 The posterior part, or chyle-stomach, is thin-wallecj, 

 and usually constitutes the longest part of the diges- 

 tive tract; it may be of equal calibre throughout, or 

 dilated anteriorly, as in Isopods. Into it opens the 

 , which may be either simple caecal pouches or as 

 a complex gland. The caeca may be one azygos 

 (Srhizopoda, Sida, Pleuromma) ; two, short (Clado- 

 cera), or long (Caprella, C'yamus); two lateral and a 

 medial (Polyphemus, Temora) ; two pair (Oniscus, 

 Gammarus, Lygidium) ; three pair (Idothea, Arga, 

 Ligia) ; four pair (Mysis, two small anterior and two 

 large posterior, Balanus), or none (Chthamalus, Coro- 

 nula, &c.) Or the caeca may be divided at their fun- 

 dus, or ramified (Argulus, Hedessa). In Apus there 

 are seven, ending in branched glandular nerves. In 

 rhyllosoma there is a pair of finely-branched caeca. 

 The second form of liver consists of numerous follicles 

 scattered over the wall of the chyle-stomach (Bopyrus 



* Sometimes so much so that the oesophagus first bends forwards, and 

 then turns sharply on itself as in Limulus. 



