J48 LOWER INVERTEBRATES. 



nectiou with the water-feet, which are simjile tentacles without suckers at their tips. 

 They make their exit between the hiteral jjhites of the ann-covering. Each of the five 

 angles of the mouth is formed of five pieces. The two halves of one or two arm-bones 

 (Lyman says two, because there are two sockets for tentacles) are modified into strong 

 mouth-frames, movably articulated and swung ajiart from each other. The extremities 

 of these mouth-frames are soldered to a jaw or inter-ambulacral piece, and to the inner 

 edges of I'ach pair of jaws is articulated a long, narrow jaw-plate, which su])]iorts a 

 variable number of processes, which :\w called teeth, and clduhtlcss serve tlie ]iui']»ise 

 of such. 



On either side of the base <if each arm are the radial shield abdve, and the genital 

 jilale below. These are joined at the maigiu of the disc, and eniineeted by an 

 adductor muscle. On the under side, in the sjiaee between the arms, are one or two 

 genital openings, parallel with and close to each arm. These, in the great majority 

 of s])ecies, enter a pecidiar sac, the genital bursa, with which the tubes from the 

 ovaries or sjiermaries communicate. ^V|>ostiilides cdusiders that these burste should be 

 regarded as respiratory sacs, as tlie\ may be x en to .alternately contract and dilate. 

 Each inner angle of the mouth is usually covered by a ])late, called the month-shield, 

 and one of these usually serves as the niadrejiorie body. 



The nervous and circidatory systems, and .arrangenu'nt (if the water systeui, are 

 upon the nsiuil star-fish jilan. The body cavity consists of an enlarged portion sur- 

 rounding the digestive tube, ami a fiattened ])ortion in the dorsal region. The nerves 

 have been found to contain cells with large nuclei, sduiewliat resendiliiig the jiigmeut 

 cells of vertebrates, and also delieale lilirils, u itli jiale, lii-]i(ilnr cells not c<illeeted intu 

 ganL;ii.-i. 



Tlie Oplii\u-oiilea fall into two families. In the first and larger family, Oriili:Kll).K, 

 the :ixis of each arm is encased in a greiUer (ir less number of ]ilates, tlie jirincipal of 

 which, from their position, are kiiowii as the dursal, ventral, and lateral jilates. Tlie 

 lateral plates bear a more or less numerous series of s])ines, and are usually considered 

 homologous with the ad-aml)ulacral ]>lates of the arm of an ordin.ary star-fish ; the ventral 

 and dorsal )ilates are |iviinaril\ uup.iire(l, .ami the former, at least, are peculiar to the 

 group. Mouth-shields aie .ilways present, and there .are often two other supei-fici.al 

 ])lates, the side uidUth-shields, diie on each of the outer sides of each mouth-shield. 



The ()|iliiurida' rarely ha\c more than ti\c arms, .and these ww in all cases 

 mdiranched, but in the other family, the .\strophytida', thi' five arms usually dividi' 

 and sub-divide into a very large nnniher i>f lii'iiiiehes. 'I'lie latter family is destitute 

 of the regular covering of |ilates that ]>votect the ai'nis of the Ojihiurida-, but in its 

 jilaec has a thick skin, uniler which are plates, usually of an irregular and I'lenuaitary 

 character. The arm ossicles eonsist of a \cilieal and a horizontal hour-glass-like jiro- 

 jcction, fitted one on the other. There are no spines on the sides of the arms, and 

 mouth-shields are often absent in the blanched species, in which the inadre))oric jilates, 

 sometimes one and sometimes H\e in iiuniln>r, may be found in various jiaits of the 

 lower iuter-bracliial spaces. 



The arm ossicles of the Ophiurida', according to Ludwig, are originally double, 

 the fii-st rudiment consists of two calcareous pieces symmetrically placed on either 

 side the middle Une of the arm; each triangular piece is formed of three rays, two 

 directed orally, the third ab orally; the latter increases considerably in length, and the 

 two others gradually become fused together. Till a kite stage of growth, there is in 

 the middle of the ossicle a space witli concave sides. 



