xvi ECHINODEKMATA 565 



The origin of the Holothuroidea is most plausibly explained as a 

 further development of primitive Echinoidea in the direction of 

 haunting crevices. Just as the snake has lost its limbs in order to 

 become adapted to wriggling through crevices, so the Holothuroid 

 has lost its spines and reduced its plates to vestiges in order to 

 render its body sufficiently flexible to worm its way through narrow 

 openings. Synapta forms appropriately the end term in this series 

 of modifications, for, in this form and its allies, wriggling through 

 crevices has become changed into burrowing into sand and mud. 



If the above interpretation of the developmental history of 

 Echinodennata be accepted, and it may be fairly claimed to be in 

 consonance with all the facts so far known, we may draw some 

 interesting conclusions as to the modifications which the record of 

 ancestral history, as embodied in ontogeny, has undergone. We see 

 that a stage of development, viz. the fixed stage, may be completely 

 omitted, as is the case in Ophiuroidea, Echinoidea, and Holothuroidea, 

 and in this case the organ of fixation, the prae-oral lobe, is only 

 vestigially developed ; whilst, on the other hand, certain organs 

 belonging to the preceding stage, viz. the ciliated band and its 

 processes, are retained long after the period when, to judge from the 

 stage of development of other organs, they should have disappeared. 



A precisely similar phenomenon is seen in the retention of the 

 external gills of the salamander after all four limbs have become 

 adapted for life on land. Further, an organ which should shift from 

 one position to another by the unequal growth of surrounding parts, 

 may disappear in one place and be reformed in another, as the mouth 

 in Asteroidea and Echinoidea. It by no means follows that larvae 

 which are primitive in one respect are primitive in all. Thus the 

 Brachiolaria retains a stalk but forms a new mouth, whilst the 

 Ophiopluteus has no trace of a stalk but retains the old mouth. 

 Finally, the condition in which development culminates, viz. the 

 adult condition, tends to be reflected back to earlier and earlier 

 periods in ontogeny in the case of some organs, and this is what is 

 termed precocious development. 



From all these considerations it follows that different larvae 

 which reflect in blurred form the same ancestral history, have 

 become specifically modified; and this applies not only to larvae 

 belonging to different classes, such as the Ophiopluteus and Echino- 

 pluteus, but to the larvae belonging to minor divisions within the 

 class. Thus the larvae of Spatangoidea seem all to have the aboral 

 process, whilst those of Echinus and its allies are totally devoid of it 

 but possess ciliated epaulettes. 



Lastly, what preserves a continuous record in ancestral history is 

 the continuity of functional activity. If this be interrupted all sorts 

 of aberrancies may occur. Thus the functionless larval gut of Solaster 

 and Antedon is formed by a secondary development from the coelom, 

 and the larval stomodaeum is either not formed (Solaster} or never 

 reaches the gut (Antedon). 



