Introduction to A nimal Morphology. 353 



with compound facetted eyes on movable stalks ; the 

 three thoracic limbs modified into maxillipedes, and 

 respiration by special gills. The ccphalogaster (head, 

 thorax and abdomen) is generally covered by a large 

 calcified dorsal shield of two parts, an anterior or 

 ccphalostcgite, often prolonged medially forwards into 

 a rostrum, a posterior oxomosfcgitc. 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 arc included : 



Order i. Decapoda (Latreillc] 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, antenna, mandible (with a 3-6 jointed palpf), and 

 two maxilla;, respectively ; the three thoracic segments bear 

 three pair of maxillipedes, which overlap each other. The 

 :iir 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 chelce. The 

 second and third pair of abdominal feet have generally much 

 smaller nippers. In all these- the largest Made of the elaw is 

 internal. Tin; fourth and fifth pairs have non-prehensile 

 dactylopodites, and an; usually s. v. n-joinled. When the 

 post-abdominal segments are movable their limbs consist of 



e processes from \vh: ring on the limbs arc known a* 



t This is of use in directing food currents : ;iili. 



