LARVAL ORGANS 



275 



FIG. 125. A Deep-Sea Crinoid, Penta- 

 crinus (Isocrinus) decorus, about nat. 

 size. 



stage. Later on the young animal develops cirri and falls off 

 the end of 

 the stalk, 

 assuming 

 once more 

 an active 

 modeoflife. 

 Here we 

 recognize 

 very clearly 

 in the pen- 

 tacrinoid 

 stage of the 

 ontogeny 

 a repeti- 

 tion of the 

 stalked 

 condition 



through which the ancestors of the feather-star 

 must have passed. 



We also find amongst invertebrate animals 

 abundant examples of special larval organs. 

 The body of a caterpillar, for instance, is built 

 up very largely of such structures, and it has 

 to undergo an almost complete reconstruction 

 within the protective envelope of the chrysalis 

 before it reaches the adult condition and emerges 

 as a perfect insect. It is therefore extremely 

 difficult to interpret the caterpillar stage in 

 terms of the ancestral history. 



The ophiuroids or brittle stars (Fig. 126), so 

 commonly met with between tide-marks, have 

 remarkable, free-swimming " Pluteus " larvae 

 (Fig. 127), totally different in appearance from 

 the adult. These larvae occur in immense 

 numbers in the surface waters of the ocean. 

 They are provided with long slender arms 

 supported on calcareous rods, which probably 

 serve, in part at any rate, to protect the very 

 delicate body from the attacks of other small animals. Before 

 attaining the adult condition the larva undergoes a complex 



T 2 



