92 < OiAMOCRINUS DIOMED E. 



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 thai the greatest difficulty in regarding the Holothurians 

 as representing the primordial type of Echinoderms. lies in the very Eact 

 so strongly emphasized by Semon himself, that the ambulacra! tubes are 

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

 fortunately we know as yet no development of any llolotburian to show 

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

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

 primary tentacles of the Holothurians to the primary tentacles of other 

 Echinoderm embryos, an assumption which is not recognized by other 

 writers on the homologies of Echinoderms. Semon is mistaken in stating 

 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 the 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 figure 

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

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

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

 species; they refer merely to Fewkes's article in the .Memoirs of the Peabody Academy, Vol. I. Xo. 

 VI., 1881. 



Fewkes (Proc. Bost. Soc. Nat. Hi;t., Vol. XXIV., 188S, p. 9G) considers the primary spines of 

 embryo Echinoderms 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, 

 hut tli.- young of all Echinoderms developed from a pluteus, arc free swimming for some time aftei the 

 resorption of the pluteus. Young Stat fishes and Ophiurans can float readily with the actinal surface 

 upward for a considerable time, and young Echinoids similarly, with 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 as it is known among 

 Echini, is for progression : ami in Ccelopleurus, in Arbacia, in the Cidaridse and Echinothurise, where 

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

 The Comatulse alone can --Aim. while Ophiurans use their arms in creeping. The reticular structure 

 ol ill" -pines, allowing free absorption for water, would seem to adapt them far better for floats, as is 

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

 ral to look upon these spatulate spines as due to the nature of the reticulation of the embryonic spines, 

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

 -pines of certain Cidaridae and Arbaciadae, 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. 



t In tie- young of Ilciniaster cavernosas figured by Loven, he shows the ocular plate perforated by 

 ; terminal tentacle (Poartalesia, Plate XIV Figs. 164, Kit!;, and from homology with the Star- 

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



