March 31, 192 1] 



NATURE 



^ZZ 



position, persisted as the mouth of the adult, and 

 this probably represents an ancestral stage of all 

 echinoderms. 



The opening of the hydrocoel (the subsequent 

 water-ring) to the exterior has a strange history. 

 First a prolongation from the incipient hydrocoel 

 is cut off as a canal, called parietal. This effects 

 an outer opening (pore No. i), which afterwards 

 closes. At a later stage a fresh canal (stone- 

 canal) grows out from the water-ring and opens 

 into the remains of- the closed parietal canal ; and 

 a fresh pore (No. 2) opens from outside into the 

 same parietal canal. Thus the water-ring is for 

 the first time connected with the outer medium. 

 Dr. Mortensen regards pore No. 2 as homologous 

 with the madrepore openings in other echinoderms, 

 and he is probably right. Yet he also regards it 

 as identical with pore No. i when speaking of 

 its "temporary obliteration," although several 

 ancient stalked echinoderms, in various classes of 

 Cystidea, have two neighbouring but distinct open- 

 ings, one of which is plainly a water-pore (pore 

 No. 2), while the other has been regarded as the 

 opening of the parietal canal (pore No. i). These 

 facts suggest that pore No. i never was a water- 

 pore, but may have been a gonopore, or excretory, 

 or both. Close to the parietal canal, in the dorsal 

 mesentery, is a group of cells regarded by Dr. 

 Mortensen, following Russo, as a primary gonad 

 homologous with the genital organ of Holo- 

 thurians. Dr. Mortensen believes that this struc- 

 ture is soon absorbed, and that the genital organs 

 arise as a new structure connected with the axial 

 organ. If the original cells are not carried over 

 into the subsequent gonad, their genital nature 

 seems unproved. It is not impossible that some 

 persistence may ultimately be detected. Mean- 

 while, their position harmonises with the sug- 

 gestion that the genital products were set free 

 mto the parietal canal and emerged through its 

 pore (No. i), which was the gonopore of the 

 cystids. 



The crinoid larva normally fixes itself by its 

 anterior end, and the vestibule then moves up 

 towards the future oral end of the pentacrinoid. 

 Thus the cup of the crinoid is erect on a straight 

 stem with a flattened base (like a wineglass). In 

 Tropiometra the suctorial disc is weak; many 

 embryos fall to the bottom, and the migration of 

 the vestibule is hindered by pressure; thus the 

 crinoid grows with a curved stem. May not such 

 a cause have initiated the evolution of the curved 

 stems and pendent crowns in Herpetocrinus and 

 the Calceocrinidae? 



Dr. A. H. Clark has maintained that the anal 

 NO. 2683, VOL. 107] 



plate of comatulids represents the radianal (the 

 lower half of the right posterior radial), while 

 others have homologised it with the other anal 

 {x) of palaeozoic inadunate crinoids. Dr. Morten- 

 sen believes that his observations fully confirm 

 the latter view, and, further, indicate that x was 

 derived by vertical fission from the upper half of 

 the right posterior radial. This seems a sound 

 hypothesis, and it really differs very little from 

 that implicit in the tentative term "brachianal." 

 Opposed to all these is the fourth hypothesis, that 

 the anal x was an entirely new growth. It must 

 be left to fossils to decide. 



Infrabasals have been detected in Antedon medi- 

 terranea and A. adriatica, but not in .4. bifida; 

 Dr. iMortensen, however, ahvays finds them in 

 that species. Isometra, Fiorometra, and Thau- 

 matometra are the only forms in which he has not 

 found them. In all cases the first cirri are radial 

 in position, as they are in all Crinoidea Dicyclica, 

 whether the infrabasals have atrophied out of 

 existence or no. It is not realised that the position 

 of the cirri depends on the position of the nerves 

 of the aboral system, a position originally 

 governed, no doubt, by the presence or absence 

 of infrabasals, but maintained without regard to 

 the subsequent history of the skeleton. 



Dr. Mortensen observes that in the growth of 

 these crinoids the pinnule-bearing brachials no 

 longer make their first appearance as axillaries. 

 Since he admits, however, that each pinnule has 

 the morphological value of an arm, the brachials 

 which bear them are, morphologically, axillaries. 

 That the pinnules did originate as arm-branches 

 is confirmed by palaeontological evidence. 



Palaeontologists have long since given up 

 Loven's attempt to homologise the elements of the 

 crinoid cup with the apical plates of echinoids. 

 It is satisfactory to find Dr. Mortensen led to 

 the same conclusion. But that is a big subject. 

 We have said enough to show that for this fruit- 

 ful memoir our Danish colleague and his American 

 publishers deserve the thanks of morphologists, 

 embryologists, and palaeontologists. 



F. A. Bather. 



Electrical Theory and Relativity. 



The Mathematical Theory of Electricity and Mag- 

 netism.^ By J. H. jeans. Fourth edition. 

 Pp. vii-t-627. (Cambridge: At the University 

 Press, 1920.) 245. net. 



SINCE the third edition of this volume was pub- 

 lished in 191 5, the theory of relativity has 

 been developed. It is now recognised that' Max- 



