ECHINODERMA GENERAL DESCRIPTION 19 



Further, if our present theory be correct, we must suppose that 

 the larval history of the Holothurians has been exceedingly com- 

 pressed ; so that, to take but one point, the development of the 

 straight larval gut in^o the coiled gut of the adult takes place, not 

 by migration of the mouth and associated organs, but by lengthen- 

 ing and twisting of the gut itself. It is noteworthy that the two 

 lateral radii of the trivium with their nerves and muscles and tube- 

 feet, as well as the oral tentacles to which they eventually give rise, 

 develop much more slowly than the three other radii. Those are 

 the three radii which are assumed in the above account to be 

 homologous with the original three radii of the primitive Pel- 

 matozoan (cf. Fig. IX.). 



It therefore appears that the Holothurian stock branched off 

 from the Pelmatozoa before complete pentamerous symmetry of 

 the hydrocoel and associated organs had arisen, before any definite 

 calycinal system had developed, while the gonads were still a 

 simple strand opening to the exterior by a single posterior gono- 

 pore. The diminution of the skeletal elements did not favour 

 their preservation as fossils. Their spicules indeed are found in 

 the rocks from at least the Carboniferous downward, but if we 

 except the problematic Sphaerites, Quenstedt (1852, non Dufts), no 

 fossil Holothurian is known. The class was perhaps an early 

 offshoot from the Edrioasteroidea. This theory explains how it is 

 that the Holothurians are primitive in so many characters, although 

 the most specialised in others ; they are primitive as regards 

 Pelmatozoic structure, specialised as regards Eleutherozoic, 



Symmetries. The radial symmetry due to the fixed phylo- 

 genetic stage is usually pentamerous. Hexamerous symmetry 

 was independently acquired by some Cystidea. Variation from 

 pentamerism may arise suddenly (discontinuous meristic variation 

 of Bateson), producing hexamerous or tetramerous individuals, or 

 species, or genera, according as the sport becomes fixed. There 

 may also be a duplication, or further multiplication of radii, as in 

 the ten-rayed Promachocrinus, or an intercalation during growth, as 

 in the many-rayed Labidiaster ; this is a different thing from the 

 branching of a radius, such as occurs in Crinoids, Astrophytidae, 

 and elsewhere. Again there may be variation by gradual atrophy 

 of one or more radii, as in Calceocrinidae, and some heart-urchins 

 and Holothurians. In spite of these variations, it is generally 

 possible to divide the body of an Echinoderm, by planes passing 

 through the ambulacra from the long or main axis, into approximately 

 corresponding portions, " antimeres," normally five. These planes 

 mark the radii, or better perradii, since the terms ray and radius 

 have been used loosely. Organs bisected by them are "perradial" ; 

 such are invariably the main ambulacra! vessels, the arms of 

 Stelleroidea and brachia of Crinoidea, with their included organs. 



