410 INVEBTEBBATE MOBPROLOOY. 



4. Order Decapoda. 



In the Decapods the carapace is well developed, covering 

 in the thorax completely (Fig. 162), the segments of that region 

 of the body fusing with it dorsally, so that a perfect cephalo- 

 thorax is present. The antennules generally possess two 

 terminal multiarticulate flagella, and the antennae frequently 

 lack the scalelike exopodite which occurs in other groiips 

 (e.g., Schizopoda). In the second maxillae the exopodite is 

 transformed into a platelike structure which, swinging to and 

 fro, serves to renew the water in the branchial chamber lying 

 between the lateral portions of the carapace and the Lody- 

 walls. On account of this action this appendage is usually 

 spoken of as the scaphognathite. The three anterior thoracic 

 appendages are maxillipeds, the third one frequently becom- 

 ing almost limblike, a characteristic which distinguishes the 

 five posterior pairs of appendages which are adapted for walk- 

 ing and are hence termed the pereiopods. They lack all 

 traces of exopodites, though usually bearing epipodites and 

 branchiae, and a certain number of the anterior ones ai'e fre- 

 quently chelate, thus serving for the prehension of food. The 

 number of the pereiopods has suggested the name given to 

 the order. The abdominal appendages are sometimes want- 

 ing or very rudimentary, but when present are biramous swim- 

 ming-feet and are hence termed pleopods — a term equally 

 applicable in some other groups. 



The branchise lie entirely within the branchial chamber 

 and are developed in connection with the thoracic append- 

 ages. They may be seated upon the basal joints of the ap- 

 pendages (podobranchia), or upon the joint between the ap- 

 pendage and the body-wall (arthrobranchia), or finally upon 

 the body-wall itself (pleurobranchia). All three kinds may 

 occur on the same segment, so that the entire number of gills 

 may be much greater than that of the appendages, amount- 

 ing in the Lobster to no less than twenty' in each branchial 

 chamber. 



The heart is a short saclike organ lying in the thorax and 

 possessing as a rule three pairs of ostia, one pair being situ- 

 ated on the dorsal surface, one upon the sides, and the third on 



