92 CALAMOCRINUS DIOMED-??:. 



derms, to satisfy us that the position of the abactinal system of the young 

 Echinoderm is not limited to one of the water tubes. 



It seems to me that the greatest difficulty in regarding the Holothurians 

 as representing the primordial t^-pe of Ecliinoderms lies in the very fact 

 so strongly emphasized by Semon himself, that the ambulacral tubes are 

 interradial, as he considers them, and the primary tentacles radial. Un- 

 fortunately we know as yet no development of any Holothurian to .show 

 the manner in which the ambulacral tentacles are related to tlie ambula- 

 cral canal during their growth. Semon assumes the homology of the fiist 

 primary tentacles of the Holothurians to the primary tentacles of other 

 Echinoderm embryos, an assumption which is not recognized by other 

 writers on the homolo<>'ies of Echinoderms. Semon is mistaken in statino; 

 that the primary tentacles of the embryo Echinus disappear. They are 

 retained in the genera which have been studied, namely, Strongylocen- 

 trotus, Arbacia,* Abatus,t and Brissopsis, so that the essential character 



* The figure of a young Arbacia given in the Revision of the Echini (p. 735. Fig. 69) shows the 

 odd terminal tentacle of the young Echinoderm as seen from tlie actinal side at the time when there 

 are only three pairs of tentacles to each ambulacrum, and it is impossible to make out the arrangement 

 of the coronal plates either in the ambitus or on the abactinal side. 



Garman and Colton (.Studies from Biol. Lab. of Johns Hopkins Univ., Vol. II. p. 247) give a figi\re 

 (Plate XVIII. Fig. 9) of a young Arbacia in whicli the four anal plates are well developed, and five of 

 the plates of the genital ring very faintly indicated. They seem never to liave seen the Revision of 

 the Echini, nor is there any reference made to the many figures given by Muller of the Mediterranean 

 species; they refer merely to Fewkes's article in the Memoirs of the Peabody Academy, Vol. I. No. 

 VI., 1S81. 



Fewkes (Proc. Bost. Soc. Nat. Hist., Vol. XXIV., 1888, p. 9G) considers the primary spines of 

 embryo Echinodei-ms as special organs representing the survivals of swimming organs in some prob- 

 lematical ancestral free swimming Echinoderm. 



It is well known to all those who have used the pelagic fishing net, that not only young Arbacia, 

 but the young of all Echinoderms developed from a pluteus, are free .swimming for some time after the 

 resorption of the pluteus. Young Stai fishes and Ophinrans can float readily with the actinal surface 

 upward for a considerable time, and young Echinoids similarly, witli their gigantic tentacles swollen 

 with water, present sufficient surface to float quite heavy tests. The floating of Starfishes, as also of 

 young Holothurians, is due to the same cause. The use of the spines, as far .is it is known among 

 Echini, is for progression ; and in Coelopleurus, in Arbacia, in the Cidaridaj and Echinothuriie, where 

 they are specially developed, they are used for ambulatory purposes. The same is true of Opliiurans. 

 The Comatulae alone can swim, while Ophiuraus use tlieir arms in creeping. The reticular structure 

 of the spines, allowing free absorption for water, would seem to adapt them far better for floats, as is 

 the case in the appendages of Globigeriuae, for instance, than for primordial paddles, it is more natu- 

 ral to look upon the.se spatulate spines as due to the nature of the reticulation of the embiyonic spines, 

 which when it becomes excessively developed in a lateral direction would give rise to the flattened 

 spines of certain Cidarid:B and ArbaciadiE, while when developing regularly they form more or less 

 cylindrical radioles. Furthermore, none of the early types of Crinoids possess anything resembling 

 these primordial swimming organs. 



•j- In the young of Ilemiaster cavernosns figured by Loven, he shows tlie ocular plate perforated by 

 the odd terminal tentacle (Pourtalesia, Plate XIV Figs. 16i, 160;, and from homology with the Star- 

 fishes it lias been the custom to denote them as ocular plates. 



