I. CRUSTACEA: STOMATOPOUA. 



429 



The Mysidid.'E arc the most widely distributed, several species of Mijsis 

 (fig. 434) occurring on our coasts and one in the Great Lakes. In these the 

 endopodite of the sixth abdominal appendage contains an otocyst, with a 

 calcic fluoride otolith. Other families are the Euphausiid.e and Lopho- 

 GASTRiD^ of the deeper seas. 



Order II. Stomatopoda. 



In structure of the cephalotliorax these forms, known as mantis 

 shrimps (from a resemblance to the insect, the jiraying mantis), 

 have not advanced as far as the schizopods, since the hist three 

 thoracic somites remain free and are not covered by the carapace. 



Tig. 435.— .S'<jW!(Za mantis, at, «(', first and second antenna? ; /, sixth abdominal feet : 

 k, gills; p, schizopodal thoracic feet; ]>r, pr', raptorial feet; ps, pleopoda ; s<(. 

 telson. 



The appendages, liowever, are more differentiated, since only the 

 throe posterior thoracic feet are biramons and natatory. The 

 four in front of these are prehensile and bear a pincer formed of 

 the last two joints, the last being slender and usually toothed and 

 closing in a groove of the penult joint like a knife Ijlade in the 

 handle. The first of these raptorial feet are the largest atid are 

 used in capturing fishes, etc. Since the thoracic feet are of little 

 service for locomotion, the abdomen is long and stout, especially 

 the caudal fin. The five anterior abdominal feet boar the gills, and 

 correspondingly the elongate heart with many ostia extends into 

 the abdomen. The transparent pelagic larva^ were formerly re- 

 garded as adults and described as Alima and Eridithus. Squilln 

 emjmsa lives on our east coast, Gonodactylvf: in Florida. They are 

 burrowing animals and deposit their eggs in their holes. 



Order III. Decapoda. 



The Decapoda is the most important group of Crustacea, since 

 it contains the shrimj)S, lobsters, crayfish, and crabs. It agrees 

 with the 8chizop)oda in having a cejihalothorax composed of thirteen 

 fused somites, but differs in the structure and function of the 

 thoracic extremities. Only the last five pairs (whence the name 

 Decapoda) are locomotor. These lose the exopiodite during de- 



