ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 199 



Later on, however, they undergo a considerable regressive 

 metamorphosis, Avhile the abactinal skeleton of the arms 

 develops very rapidly, as the water -vascular stems extend 

 outwards from the disc, bearing with them the odd terminal 

 tentacle. This mode of growth, with some slight modifica- 

 tions, is common to the Asterids and Ophiurids, the arms of 

 which, as of the Crinoids, may be regarded as an extreme 

 development of the primary tentacular caeca borne upon the 

 water-vascular ring of the larva, which becomes much en- 

 larged and acquires a calcareous skeleton. The water- 

 vascular ring of the Holothurian embryo also bears five tenta- 

 cular caeca, but the water-vascular trunks indicating the 

 five antimera, are not formed (when present) by these caeca 

 being carried outwards away from the ring, as is the case in. 

 the Starfishes ; for they appear separately as five other diver- 

 ticula from the water-vascular ring, alternating with the 

 tentacular rudiments. 



Gotte assumes that these caeca are of primary importance, 

 and occupy a similar position in all Echinoderms. He is 

 consequently led to regard the Starfish arms, with their 

 primitive terminal tentacles, as homologous with the 

 branching tentacles of the Holothurians. In the same way 

 he compares the intertentacular antimers of the Holothu- 

 rians to the interradial antimers of the Crinoids ; for these 

 which contain the basal and oral plates, alternate with the 

 tentacle-bearing arm rudiments, that ultimately attain so 

 great a development and obliterate or obscure the primary 

 interradial skeleton. 



I regret that I am unable to accept Gotte's views. He 

 brings forward no argument in their favour, except, of course, 

 one of time, namely, that the five caeca which first appear 

 as outgrowths of the water-vascular ring, develop in the 

 Holothurians into the branched oral tentacles, and in the 

 Starfishes into the large terminal tentacles at the ends of 

 the arms. Further, if Gotte's views are correct, the con- 

 clusions which naturally follow from them are completely at 

 variance with many facts of Echinoderm morphology. He 

 regards the condition of the Ech'mi as essentially similar to 

 that of the Holothurians, though the ambulacral areas are 

 not completely homologous in the tAvo groups, owing to the 

 presence of interambulacra in the Urchins, and to slight 

 difi"erences in the mode of origin of the water-vascular trunks. 

 In the Urchins the " urspriinglichen Tentakelanlagen werdeu 

 nicht in Arme fortgesetzt, deren Entwickelung eine beson- 

 dere aborale Strahlgliederung unterdriickte und die damit 

 alternirende orale Gliederung zur ausschliesslich herrschen- 



