CLASS CRUSTACEA. 29 



segment consists of an upper and a lower arch of shell, 

 joined to one another at the sides, and sending down- 

 wards a plate from the point where they join (fig. 13, 2). 

 In theoiy, also, each segment carries a pair of " appen- 

 dages," which ought to be composed of two branches 

 springing from a common base. In some cases, as in the 

 tan ("abdomen") of the Lobster, the actually existing 

 segment really does conform to this theoretical type ; but 

 in other cases the segment may be very variously modified, 

 and the appendages of the segment, in particular, assume 

 the most different forms in difi'erent parts of the body. 

 In practice, also, some of the segments are so amalgamated 

 and consolidated with one another as to render their 

 recognition a matter of great difficulty. 



It will, however, greatly help us in studying the Lob- 

 ster to remember that the body of the animal is really 

 composed of a number of segments (twenty-one), each of 

 which is constructed upon the theoretical plan or type 

 just mentioned, and each of which may carry a single 

 pair of appendages of some kind or other. 



If we look at the Lobster from above (fig. 13, i), we 

 see at once that the body is very plainly divided into two 

 parts, which would familiarly be called the "head" and 

 "taU." The "head," as we should call it, is in truth 

 composed of fourteen rings all amalgamated together, and 

 covered above by a great shield or buckler (c«). The 

 fii-st seven of these segments belong to what is actually 

 the head, whilst the hinder segments belong to what is 

 properly called the trunk or "thorax" (Greek, thorax, a 

 breastplate) ; and the head-shield shows this division by 

 a transverse groove on its upper surface. On the other 

 hand, the so-called " tail " is composed of a number (seven) 

 of quite distinct rings or segments, which are movably 

 jointed together, and which collectively constitute what 

 is termed the "abdomen" (Latin, ahdo, I conceal). 



The Lobster is therefore really composed of twenty-one 

 rings or segments, seven of which are free and movable, 

 and constitute the abdomen, whilst the remaining fourteen 

 are amalgamated together, but really belong, seven to a 

 thorax and seven to a true head. 



