438 



EMBRYOLOGY 



enterocoelic body cavity, the intestine, etc. — then form a part 

 of the permanent star ; the mouth is said to persist, whereas 

 the anus disappears. Upon the completion of these processes, 

 which result in the establishment of the- permanent shape 

 of the animal, the calcareous skeleton of the Plnteus disin- 

 tegrates. The rods break up into pieces; as a result of this, 

 the arms collapse, and the skeleton, together with the larval 

 body, appears finally to be resorbed by the young Ophiuran. 



'"»l^^^ 



Fig 211.— Pluteus larva with the fun- 

 datuent of the Ophiuran (after Joh. 

 Mulleb). The rods of the larval skele- 

 ton are dark. 



Like the arms of the starfish, 

 those of the Ophiuran grow at the 

 tips, with the exception of the 

 terminal pieces, which correspond 

 to the skeletal i^ieces first formed 

 on the dorsal surface. Therefore 

 the new pieces are interpolated 

 between the terminal pieces and 

 the adjacent ones. The skeletal 

 parts thus follow a law quite 

 similar to that of the feet, the de- 

 velopment of which always takes 

 place between the (terminal) ten- 

 tacle and the pair of feet next to 

 it. The origin of the arm-plates 

 is interesting ; according to LuDWia 

 (No. 34), they result from the 

 fusion of two calcareous plates ly- 

 ing on either side of the median 

 line of the arm. 



Echinoidea. — According to Mktschnikoff's description 

 (No. 37), a difference exists between the metamorphosis of 

 the Echinoidea and that of other Echinoderms, inasmuch as 

 there is developed an invagination of the larval skin, at the 

 bottom of which the earliest fundament of the body of the 

 sea-urchin makes its appearance. Thus it happens that 

 the earliest fundament is not exposed, but is covered by a 

 fold of the larval skin as though by an amnion. Since, how- 

 ever, the larval skin here too becomes directly converted 

 into that of the sea-urchin, this difference does not appear 

 to us to be in any way important. 



The processes of the metamorphosis of the Plnteus into 



