Morphology of the Biastoidea. 295 



spiracles round the mouth, he is in reality widening the gap 

 between a Pentremite and an Urchin. The ambulacral ten- 

 tacles of an Urchin are supplied by lateral branches of a 

 single median water-vessel, which does not open to the exte- 

 rior in the neighbourhood of the mouth, but unites with its 

 fellows into the oral ring. These features are eminently 

 characteristic of all Echinoderms and not merely of the Echi- 

 noidea, in which group Mr. Hambach finds the nearest allies 

 of Pentremites ; though from his description of the tentacular 

 apparatus of the latter type it must be entirely different from 

 that of any living Echinoderm. 



I believe, however, not only that Pentremites and the Blas- 

 toids generally had radial vessels and an oral ring homologous 

 with those of the other Echinoderms, but also that their 

 hydrospires are represented in some recent types, though not 

 by the tentacular apparatus, as supposed by Mr. Hambach. 

 For it appears to me that Ludwig * was not far wrong in his 

 comparison of the elongated slits at the sides of the ambulacra 

 of Orop>hocrinus to the ten interradial clefts on the under side 

 of the disc of Ophiurids, which lead into the genital bursa?. 

 He says, for example, " Meine Ansicht griindet sich auf die 

 Uebereinstimmung in der Lage der Hydrospiren der Blastoi- 

 deen mit den Bursas der Ophiuriden, sowie auch auf die in 

 beidenOrganen in gleicherWeise vorkommende Faltenbildung 

 an der der Leibeshohle zugekehrten Seite." 



If then, as I believe to be the case, the hydrospires of 

 Orophocrinus correspond to the genital bursa?, of an Ophiurid, 

 this view may be extended to all the Blastoids ; and it is 

 surely a more rational one than that which postulates the 

 existence of a tentacular apparatus at the sides of the ambu- 

 lacra of Pentremites, with altogether different morphological 

 characters from the water-vascular system of other Echino- 

 derms. Then, again, the genital slits of the Ophiurids are 

 ventral in position as the hydrospiral clefts are in the Blas- 

 toids, and not dorsal like the ovarian apertures of the Echini, 

 to which Mr. Hambach compares the spiracles of the Blastoids. 

 This is only one of many anomalies which appear throughout 

 his comparison of an Echinus and a Pentremites. He for- 

 merlyf spoke of the " oroanal " surface of a Pentremite as 

 being on the dorsal part of the shell ; but he now tells us J, 

 " That portion which I had termed ' dorsal half ' should per- 

 haps have been called better ' apical ' or l ambulacral system j ' 



* " Beitrage zur Anatomie der Ophiuren," Zeitschr. f. wiss. Zool. 

 Bd. xxxi. 1878, p. 388. 

 t Trans. St. Louis Acad. vol. iv. p. 151. t Ibid. p. 542. 



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