148 LOWER INVERTEBRATES. 



nectiou with tho watev-feet, which iiri' simple tcntack's witJiout suckers at their tips. 

 They make their exit between tlie hitcral i)lates of the arm-co\ering. Each of the five 

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

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

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

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

 edges of each pair of jaws is articulated a long, narrow jaw-plate, which supports a 

 variable number of processes, wliich are called teeth, and doubtless sei-ve the purpose 

 of such. 



On either side of the base of each arm are the radial shield above, and the genital 

 plate below. These are joined at the margin of the <lisc, and connected by an 

 adductor muscle. On the under side, in the sj)ace between the arms, are one or two 

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

 (jf species, enter a peculiar sai', the genital bursa, with which the tubes from the 

 ovaries or spermaries communicate. Apostolides considers that these burs* should be 

 i-egarded as respiratory sacs, as they may be seen to alternately contract and dilate. 

 Each inner angle of the mouth is usually covered by a plate, called the mouth-shield, 

 and one of these usually serves as the madreporic body. 



The nervous and circulatory systems, and arrangement of the water svstciii, are 

 upon the usual star-fish plan. The body cavity consists of an enlarged portion sur- 

 rounding the digestive tube, and a flattened portion in the dorsal region. The nerves 

 have been found to contain cells with large nuclei, somewhat resembling the pigment 

 cells of vertebrates, and alsn delicate fibrils, with pale, bi-polar cells not collected into 

 ganglia. 



The Opliiuroidea fall into two families. In the first and larger family, Ophiubid^e, 

 the axis of each arm is encased in a greater or less number of plates, the principal of 

 which, from their p<isition, are known as the dorsal, ventral, and lateral plates. The 

 lateral plates bear a more or less nmnerous series of spines, and are usuallv considered 

 homologous with the ad-ambulacral plates of the arm of an ordinary star-fish ; the ventral 

 .and dorsal plates are ]uaniarily un])aired, and the former, at least, are peculiar to the 

 group. ]Mouth-shields are always present, and there are often two other snjierficial 

 plates, the side mouth-shields, one on each of the outi'r sides of each mouth-shield. 



The Ophiurida^ rarely ha\e more than fi\'e arms, and these arc in all eases 

 unbranched, but in the other family, the Astrophytidie, the five arms usually divide 

 and sub-divide into a very large number of branches. The latter family is destitute 

 of the regular covering of plates that protect the arms of the Ophiurida;, but in its 

 place lias a thick skin, under which are plates, usually of an irregular and elementary 

 character. I'he arm ossicles consist of a vertical and a horizont.al hour-glass-like pi-o- 

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

 mouth-shields are often absent in the branched species, in fl'hich the madreporic plates, 

 sometitnes one and sometimes five in number, may be found in various parts of the 

 lower inter-brachial spaces. 



The ai-m ossicles of the 0])hiurida% according to Ludwig, are originally double, 

 the first rudiment consists of two calcareous pieces symmetrically placed on either 

 side the middle line 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 late stage of gi-owth, there is in 

 the middle of the ossicle a space with concave sides. 



