CRUSTACEA. 315 



In the malacostracous Crustacea the mouth is closed by a small 

 and simple membranous labrum above, by a bifid labium below, and 

 by a pair of mandibles and maxilles at the sides; the mandibles 

 are the legs or jointed appendages of the fourth cephalic segment ; 

 the maxilles are the same appendages of the fifth. The mandibles 

 are worked by large muscles arising from the cephalo-thorax : their 

 internal border is hard and often toothed ; they support palpi. The 

 maxilles are weaker, softer, and without palpi. In the common 

 crab you will observe behind and exterior to the maxilles a third 

 pair of jaws, which have their principal part terminated by a cutting 

 edge, and are without palpi : the two following pairs of jaws are dis- 

 tinguished by their articulated feeler or palp and by their flabelliform 

 appendages, which penetrate into the interior of the branchial cavity; 

 the maxillary plate is armed with teeth; the sixth and last pair of 

 jaws has the maxillary plate so much expanded as to cover and 

 protect the whole oral apparatus : it has likewise a palp and flagellum 

 articulated to its base. All the foregoing maxillary organs are 

 modifications of entire limbs, which are thereby translated from the 

 locomotive series : the jointed legs so metamorphosed belong to the 

 last four cephalic, and the first two thoracic, segments. 



The alimentary canal is most simple in the suctorial Crustacea, in 

 which it presents no noticeable difference from that in the Epizoa ; 

 the hepatic appendages are however more localised and better de- 

 veloped. 



In the Limulus the mouth is situated nearly in the centre of the 

 inferior surface of the great cephalo-thoracic segment ; the oesophagus 

 is continued from it in a very unusual course forwards, and expands 

 into a stomach, which is situated at the anterior part of the head. 

 This organ is abruptly bent upon itself upwards and backwards, 

 and is continued by a gradual diminution of diameter, as appears 

 upon an external view, into the intestine, which passes backwards 

 with a slight vertical bend to the base of the penultimate abdominal 

 segment. When we examine the interior of the alimentary tract, the 

 distinction between the stomach and intestine is effected, as Van der 

 Hoeven has shown, by a conical valvular pylorus, which projects 

 into the commencement of the intestine. The stomach is lined by 

 a very dense and corrugated horny membrane. The hepatic mass, 

 composed of contorted slender caeca, which, with the generative 

 glands, fills the greater part of the cephalo-thoracic cavity and also 

 extends into the abdomen, pours its secretion into the commencement 

 of the intestine by two ducts on each side.* 



Amongst the smaller Entomostraca Lynceus departs most from 



* Prep. No. 477. A. 



