STAR-FISHES. 



155 



in the long-armed star-fishes are interradial, and are homologous with those which fill 

 up the sjDace between the arms in the pentagonal species, the ovaries are actually 

 interradial or interambulacral in position, as in ophiuroids. The eggs pass out by a 

 pore on each side of the base of the arms, situated between two plates, and difiicult to 

 detect. 



The embryo of a star-fish is usually a free-swimming animal with arms and ciliated 

 bands, and has been called a Brachiolaria, or, in other forms, Bipinnaria. The 

 Brachiolaria, when it has attained its full development, has thirteen arms — a medial 

 anal pair, a dorsal anal pair, a ventral anal pair, a dorsal oral pair, a ventral oral pair, 

 an odd anterior arm, bearing an odd brachiolar arm, and a pair of smaller brachiolar 

 arms connected with the oral ventral pair of arms. The brachiolar arms have wart- 

 like appendages at the tip, whereas all the other arms are surrounded by chords of 

 vibratile cilia. The median anal arms appear first and are largest (in Asteracanthion 

 pallidits), and the odd brachiolar arm precedes the pair of similar arms. The adult 

 larvse move about rapidly by means of the cilia, with the oral extremity in advance. 

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

 ciliated. 



The first commencement of the growth of the young star-fish is by the appearance, 

 on the anal side of the left water-tube of the larva, of five slight folds ; while on the 

 other side of the anal ex- 

 tremity appear five lime- 

 stone rods. The folds are 

 the first rudiments of the 

 rows of suckers, the rods 

 of the aboral skeleton. 

 The rays are indicated as 

 lobes upon the growing 

 dorsal part of the disc 

 while the suckers are still 

 widely separated from 

 them. The young star- 

 fish, thus growing on the 

 opposite surfaces of the 

 two water- tubes, soon 

 loads down the anal end 

 of the embryo, the larva 

 then becomes sluggish, the 

 body changes from trans- 

 parent to cloudy and 

 opaque, the arms contract 

 and become constricted 

 into cells, and soon noth- 

 ing remains but the brach- 

 iolar arms, brought close 

 to the young star-fish by the shrinkage of the body. These finally follow the rest. 

 Not a single part is dropped off, the whole of the larva passes into the star-fish. The 

 first steps subsequent to this resorption are the approach of the oral and aboral sides 

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



Fig. 137. — Oral surface of very young star-flsh. 



