SYNAPTA VIVIPARA. 81 



terranean mode of life. The absence of anything, therefore, in their manner of life to 

 cause degeneration is by no means proven and will hardly stand as a good test for con- 

 sidering the Synaptas primitive. The statement that there is no structure developed in 

 the young Synapta which does not appear in the adult is completely refuted by the 

 careful observations of Ludwig and Barthels ('91) on the absence of radial water-canals 

 in the Synaptidae. Since all observers are agreed that radial canals are developed in the 

 embryo of S. difjitata, it is clear that we have here a most important structure lost in the 

 adult. .For these reasons, it seems to me that Semon's view is no longer tenable. 

 Cuenot's ('91) view is based chiefly on the important differences in the embryology of the 

 Synaptidae and that of otlier holothurians, but it seems to tne that he does not take suffi- 

 cient account of the important evidences of degeneration in the Synaptas. 



Ludwig's ('89-92) view appears to be the one best supported by the facts, and the 

 anatomy and embryology of >S'. vivlpara offer no little confirmatorj^ evidence. If we 

 compare its ten-tentacled stage with the hypothetical ancestor which Ludwig describes 

 for the Synaptidae, the resemblance is extraordinary, almost the only important differ- 

 ence being that the genital gland in S. vimpara is not equally developed on each side. 

 In fact, the ten-tentacled stage of Syiicqjta v'tvipara represents an actual step in the 

 development of the Synaptidae from Ludwig's hypothetical ancestor. The Jamaican 

 species is bej'ond doubt a highly modified form, and, though in some respects more highly 

 organized than other Synaptas, in certain particulars, degeneration has gone further. 

 Differing from other holothurians in its manner of life and its mode of reproduction, it 

 has undergone various modifications to fit it for the changed conditions. Living in sea- 

 weed near the surface of the water, it has developed pigment in its skin to a marked 

 degree, and at the same time has acquired additional sense-organs in the eyes at the base 

 of the tentacles, and an increased innervation of the oral disc. In conformity to its 

 changed mode of reproduction, important changes have taken place in the structure of 

 the genital gland, openings have appeared in the walls of the rectum to connect the body- 

 cavity with the exterior, while the stone-canal has retained or has acquired secondarily 

 its original connection with the exterior. During the progress of these specializations, 

 the same causes have led to degeneration in other particulars. The changed mode of 

 reproduction has modified the genital duct, so that its lumen is no longer open to the 

 ova, and it no longer has an obvious opening to the exterior. The changed mode of 

 life has caused a greater concentration of the water-vascular system around the mouth 

 and a consequent further degeneration of the radial canals, so that they no longer appear 

 as such even in the embryology, but tentacles develop directly from the secondary out- 

 growths of the hydrocoel. And, furthermore, the mid-ventral outgrowth has degenerated 



