STAB-FISHES. 



155 



in the lonji-armed star-fishes are interrarUal, and are homologous with those which fill 

 u]i the sjiaee between the arms in the pentagonal s]>eeies, the ovaries are actually 

 iiiterradial or iiitoi-ambulaeral in ]iosition, as in ojihiuroids. The egus jiass out by a 

 |jure on eacli side of the liase of the arms, situated Ix'tweeu two )ilatcs, .-uid difficult to 

 detect. 



The eniljryo of a star-fish is usually a tVee-swinuning animal with arms and ciliated 

 bauds, and has been called a Hrachiolaria, <ir, in other forms, l>i|)iiinai-ia. The 

 Brachiolnria, when it has attained its f'ldl <le\'clo|iment, has thirteen arms — a me<lial 

 anal j)air, a dorsal anal pair, a \'eiitral anal ]iair, a <lorsal oral pair, a viaitral oral ])air, 

 an odd anterior arm, bearing an odd lirachiolar arm, and a ]iair of smaller l)rachiolar 

 arms connected with the oral ventral |)air of aims, 'i'lie brachiolar arms have wart- 

 like appendages at the ti)i, whereas .all the otlier arms are surrounded by chords of 

 \ibratile cilia. The median anal arms :i]i]iear first .ami are largest (in Asturuvuiithion 

 jjdUiihix), and the odd brachiolar arm precedes the p.iii- (jf simil.ar arms. The adult 

 lar\:e ino\e about rapidly by means of the cilia, with the lu-al e\tremit\- in advance. 

 In the Biliinnaria the arms are fewer in number ami are more slender, and all are 

 ciliateil. 



The first commencement of the growth of the young star-fish is by the a])|)earance, 

 <jn the anal side of the left water-tube of the larv.a, of five slight folds; while on the 

 other side of the anal ex- 

 tiemity a|ipear five lime- 

 stcjue rods. The folds are 

 the first rudinu'iits of the 

 rows of suckers, the rods 

 of the .abcir.il skeleton. 

 The rays -.xw indicated as 

 lobes upon the growing 

 dorsal p.art of the disc 

 while the suckers are still 

 widely separ.'iteil from 

 them. The young star- 

 fish, thus growing on the 

 opposite surfaces of the 

 two water-tubes, soon 

 hiads down the anal end 

 of the embryo, the larva 

 then l)ecomes sluggish, the 

 body changes from trans- 

 p.arent to cloudy and 

 opa(|ue, tlu' arms contract 

 .and liec(]nu' constricted 

 into cells, .and soon noth- 

 ing remains but the bracli- 

 iolar arms, brought close 

 to the young star-fish by the shrinkage of the l)ody. These finally follow the rest. 

 Not a single ])art is dropjied off, the whole of the larva jiasses into the star-fish. The 

 first stejis subsequent to this resorjjfion are the a]i)iroach of the oral and aboral sides 

 by the contraction of the water-tubes, and the approach to symmetry of the at first 



-Oial .- 



young star-tisli. 



