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NATURE 



[December 8, 192 1 



tion, that forms with essentially different larvae 

 are not nearly related even when there is some 

 resemblance between the adults, may lead to dis- 

 pute when applied to particular cases. But the 

 conclusion can be tested by other characters. For 

 instance, a Mediterranean sea-urchin, Sphaere- 

 chinus granulans, approaches Strongylocentrotus 

 in certain features which have led Dr. H. L. Clark 

 to refer it to that genus. Its larva, however, is 

 found to agree with those of the Toxopneustidae, 

 not with those of Strongylocentrotus. A fairly 

 good test is afforded by the minute structures of 

 the pedicellarise, and these confirm the conclusion 

 drawn from the larvae. Although we have as yet 

 only random samples of larvae from the thousands 

 of liv'ing Echinoderm species, still observation, so 

 far as it has gone, does indicate that here is new 

 and valuable evidence of affinity. 



In directing attention to this evidence, as when 

 he demonstrated the importance of the micro- 

 scopic characters of spines and pedicellariae. Dr. 

 Mortensen has provided the systematist with a 

 new method. It is unfortunate that neither of 

 these tests can (save in some exceptional speci- 

 mens) be applied to fossils. They cannot oppose 

 the palaeontological argument, but they cannot 

 assist it ; we can only say that, for genealogical 

 trees based on the evidence of dead ancestors to 

 be correct, the arrangement of their ultimate liv- 

 ing twigs must accord with the evidence of their 

 larvae and of their anatomy generally. In so far 

 as the methods are sound, there can be no conflict 

 between them. The unsound method is that which 

 relies on only one or two characters and in addi- 

 tion too often ignores their historical development. 

 The greatet- weight should always be attached 

 to the older character. When palaeontology can- 

 not tell us directly which this is, wc may assume 

 that characters less likely to have been modified 

 by changes in the environment are the older. 

 That principle is, one supposes, consciously or sub- 

 consciously the basis of the ill-expressed statement 

 that " anatomical characters " are more impor- 

 tant than external characters. The microscopic 

 structure of a minute spine is, whether " anatomi- 

 cal " or not, such a persistent character. Simi- 

 larly, pelagic larvae, living in an environment 

 naturally subject to little change, must themselves 

 remain long unchanged, and should therefore 

 yield evidence of affinity more readily than do the 

 later stages in their varied and variable surround- 

 ings. 



It would be wearisome to repeat here the numer- 

 ous particular conclusions which Dr. Mortensen 

 bases on the resemblance of the larvae. The 

 Echinoidea yield the most results : one can recog- 

 NO. 2719, VOL. 108] 



nise a distinct larval type for each of the orders 

 Spatangoidea, Clypeastroidea, and, probably, 

 Diademoidea, and for most of the main families in 

 the regular Echinoids. The classification of the 

 Ophiuroidea is, admittedly, so unsatisfactory that 

 it has not been possible to correlate the accepted 

 families with definite larval types, but for the 

 Asteroidea this has been done to some extent. 

 The larvae of the Holothurians are not well 

 known, and the Crinoids have not yet furnished a 

 single pelagic larva. 



Fortunately the study of the larvae opens up 

 more general questions, and many will wish to 

 know whether Dr. Mortensen has anything to say 

 on the interrelations of the classes or on the 

 origin of the Echinoderms as a whole. He points 

 out that the Brachiolaria larva of starfishes with 

 its sucking disc, being found only among the more 

 specialised forms, must itself be a late develop- 

 ment, so that the sucker cannot be homologous 

 with the stalk of the Crinoids. None the less Dr. 

 Mortensen expresses his " perfect agreement " 

 with the theory which derives all Echinoderms 

 from a simple, bilaterally symmetrical creature — 

 the Dipleurula — now represented by the earliest 

 larval stage in each class, and which explains the 

 five-rayed symmetry and the torsion of the in- 

 ternal organs as due to the fixation of the 

 Dipleurula and its change into a sessile animal 

 with upwardly directed mouth and vent. Seeing 

 that this theory was published in 1900, it may be 

 held to have attained its majority. The later 

 theories of Simroth, A. H. Clark, and J. E. V. 

 Boas are treated by Dr. Mortensen with some- 

 thing as near to contempt as this courteous writer 

 ever permits himself. But as to the origin of the 

 Dipleurula, Dr. Mortensen will express no posi- 

 tive opinion. He sees no connection with the 

 Coelentera. Certainly the transition was not 

 direct, as Boas suggests; but a possible series of 

 intermediate stages was sketched by the writer 

 of the article " Echinoderma " in the " Encyclo- 

 paedia Britannica," Editions X. and XI. (1902, 

 191 1), an article always overlooked by writers out- 

 side Great Britain. 



Other matters of general interest, such as geo- 

 graphical distribution or the influence of tempera- 

 ture, must be left for those who are wise enough 

 to look for themselves at this clearly written, 

 admirably illustrated, and well produced account 

 of a long series of difficult observations. In these 

 hard times we are more than usually indebted to 

 the Carlsberg Fund, which has rendered possible 

 the preparation and the publication of Dr. Mor- 

 tensen's richly suggestive studies. 



F. A. Bather. 



