hitrodudion to Animal Morphology. 353 



with compound facetted eyes on movable stalks ; the 

 three thoracic limbs modified into maxillipedes, and 

 respiration by special gills. The cephalogaster (head, 

 thorax and abdomen) is generally covered by a large 

 calcified dorsal shield of two parts, an anterior or 

 cephalostegite^ often prolonged medially forwards into 

 a rostrum, a posterior or omostegite. The sex-opening 

 is at the ninth or eleventh post-oral somite, and all 

 the post-abdominal segments behind this have special 

 metameric shell rings. The antennae are unequal, 

 usually with divided filaments ; the liver is consoli- 

 dated, arborescent; development with a long meta- 

 morphosis. 



Two orders are included : — 



Order i. Decd-poda. {La frez'l/e) — Crabs and Lobsters ; gills 

 lamellar, in a cavity under cover of the cephalo- and omoste- 

 gite, springing from the coxopodites of the hinder pair of 

 maxillipedes, and of the five pair of ambulatory legs.* The 

 oviduct opens at the base of the third, the male organ 

 at the base of the fifth pair of legs. The segments 

 of the head from before backwards bear the eyes, an- 

 tennules, antennae, mandible (with a 3-6 jointed palpf ), and 

 two maxillse, respectively ; the three thoracic segments bear 

 three pair of maxillipedes, which overlap each other. The 

 next pair is often large, with the hinder distal angle of the 

 propodite lengthened so as to be capable of opposing the 

 dactylopodite, so as to grasp objects when the latter is flexed 

 by its powerful muscles ; these are the pincers or chelae. The 

 second and third pair of abdominal feet have generally much 

 smaller nippers. In all these the largest blade of the claAV is 

 internal. The fourth and fifth pairs have non-prehensile 

 dactylopodites, and are usually seven-jointed. When the 

 post-abdominal segments are movable their limbs consist of 



* The processes from which gills spring on the hmbs are known as 

 epipodites. 



t This is of use in directing food currents into the mouth. 

 2 A 



