METAZOA CRUSTACEA. 345 



It is not sexually mature until it is ten or eleven inches 

 long and this growth probably requires four or five years. 

 It exhibits the typical crustacean characters on a large 

 scale, and for this reason we shall give its structure in 

 greater detail, thereby summarizing in part what has 

 already been said. Many of these characters may be 

 seen in the spiny lobster, Palinurus ehrenbergi Heller 

 (No. 846), and in the preparation (No. 847, Homarus 

 americanus M. Edw.). The cylindrical body is covered 

 by a hard crust or exoskeleton and divided into two dis- 

 tinct regions, the cephalothorax and the abdomen. 



The abdomen has the more primitive characters, since 

 its six circular segments and seventh flattened segment 

 are distinct and movable. To each of the six segments 

 is attached on the ventral side a pair of appendages, the 

 swimmerets. These appendages, excepting the first and 

 sixth pairs, are composed of three parts, a basal stem and 

 two leaf-like portions. Similarity in structure suggests 

 a similarity in function, and we find that these organs are 

 used in swimming forward and in floating from below up- 

 ward. The first pair of swimmerets is modified for 

 reproductive purposes, and the last pair, which are larger 

 and stronger than the others, supplement the telson and 

 muscular abdomen in propelling the animal swiftly back- 

 ward through the water. 



The cephalothoracic segments are consolidated so that 

 it is impossible to make out their boundaries with absolute 

 certainty. They are mostly covered by a carapace which 

 bends downward on the sides, covering the branchiae or 

 gills. The number of cephalothoracic segments can be 

 inferred from the number of paired appendages attached 

 to it. 



In front of the swimmerets are four pairs of walking- 

 feet which are jointed in such a manner as to be efficient 

 organs in walking, pushing, and pulling. They are armed 

 at the end with a spike (fourth and third pairs counting 

 from the forward end) or with a movable and useful claw 



