PALINURUS OR THE SPINY LOBSTER OF BOMBAY. 363 



Ventrally however the presence of a number of transverse bars and 

 of thirteen pairs of appendages will lead you to conclude that the 

 head is composed of five, and the thorax of eight somites. 



The structure of a somite may best be studied by taking as a type 

 one from the abdomen, say the third, of which. Fig. 1 represents 



a transverse section, nearly semi- 

 \ circular in outline. The exoskele- 



ton of the segment surrounds it as a 

 ring, being hardened by calcifica- 

 tion where rigidity or protection is 

 required, and remaining soft where 

 flexibility is necessary. Dorsally 

 the hard exoskeleton forms a broad 

 arch known as the t< rgum (Latin, 

 the back) ; the lateral portions pro- 

 jecting downwards, and terminating 

 in recurved spines are called the 

 pleura (Gr. pleuron, side, rib). 

 Ventrally the ring is completed by 

 a narrow straighter bar to the outer 

 ends of which the appendages are 

 articulated. That portion of the 

 The biacK dot above it is the nerve bar between the swimming feet is 



gangion. 

 The clear space around Is the ven- called the sternum i Latin, breast- 



x = Exopodke. bone,. The very small portion of 



The shaded portions are muscles. foe bar external to the limb and 



internal to the pleuron is the epimeron (Gr. epi, upon or above ; 

 meron, the thigh). 



Between these rings of sclerites the integument is soft and flexible 

 to allow of free movement. 



The terga of the successive somites overlap one another from 

 before backwards like the tiles of a house ; their movement is restricted 

 to the vertical plane by very neat hinge-joints formed by a short peg 

 on the posterior margin close to the pleuron, which fits into a corre- 

 sponding socket on the anterior margin of the next tergum. 



The appendages are attached by soft cuticle or arthrodial mem- 

 branes to the margins of a circular opening in the ventral sclerite, 

 between the epimeron and sternum as already described. 



18 



Fig. 1.— TRANSVERSB section op 3rd. abdo- 

 minal SOMITK. 

 = CoxopodHe ) 

 B = Basipodite j Fr ' = P'Otopodite. 

 DA = Dorsal Artery. 

 E = Epimeron. 

 G = Hind-gut. 

 P = Pleuron. 

 N = Eudopodite. 

 S — Sternum. 

 T = Tergum. 

 V = feternal artery. 



