602 CLASS X. 



tailed crustaceans the pectoral shield is much developed and covers 

 the body beneath as the shell does above. The first sternal portion 

 is very large and is situated in the mid line of the body. To it 

 succeed four smaller sternal pieces, which are divided in the middle 

 more or less obviously since they consist of the union of two lateral 

 pieces. To the first piece the first pair of feet is attached, that of 

 the so-called shears or claws ; to the four following the four follow- 

 ing pairs are united. To the basal piece of the feet the lateral 

 sternal pieces (episternalia) are attached, which lie on the outside of 

 the middle pieces and fill up the truncated angles between them. The 

 most posterior portion of the abdomen in the decapod crustaceans is 

 always obviously divided into rings or segments, which are seven 

 in number, or may be fewer from the fusion of some of the rings. 

 The feet attached to these are short, commonly divided into two 

 filaments ; the hindmost ring bears no feet. These last abdominal 

 rings are usually called the tail. In the crabs they are very feebly 

 developed, and the flat tail is curved round and lies with its 

 extremity turned forwards in a groove in the middle of the pectoral 

 shield. 



The intestinal canal of crustaceans is short and straight, and 

 thus agrees with the nature of their food, which is animal. The 

 anus is situated at the extremity of the abdomen or of the tail. In 

 Limulus, however, the long styliform tail is not perforate, and the 

 anus is situated on the inferior surface of the second shield, in 

 front of the base of the tail. So is it also with the Cirripedia t 

 where the jointless tube, improperly named proboscis by some, 

 represents the tail, and where the intestinal canal opens at the base 

 of this tubular prolongation. In some lower crustaceans the intesti- 

 nal canal is nearly of the same width throughout its whole length, 

 or has in the middle or more forward an expanded part, which may 

 be considered as an imperfectly defined stomach 1 . In others an 

 obviously distinct stomach is present, which is commonly armed 

 with horny spines or with calcareous teeth. In Limulus the nar- 

 row oesophagus with longitudinal folds goes straight forwards and 



and form of the internal organs that lie beneath may be determined. See A. 

 BROGNTART and A. G. DESMAREST, Hist. not. des Crust, fossiles, pp. 73 79, and 

 DESMAREST Cons. gin. s. 1. Crust, pp. 20 23. 



1 This is the case, for instance, in Asellus (Oniscus aquaticus L.), see TREViRANUi 

 Verm. SrJir. I. s. 73, Tab. xi. fig. 64, &c. 



