gO HUBERT LYMAN CLARK ON 



that the Synaptas represent a more primitive branch of the echinoderms than and dif- 

 ferent from the true holothurians ; third, Ludwig's (*89-'92) view that the Synaptidae 

 are degenerate, pedate holothurians. Semon bases his opinion on the high development 

 of the nervous system in Synapta, the absence of anything in their manner of life to 

 cause degeneration, and the fact that no organs appear in the development of the young 

 Synapta which are not present in the adult. His own observations on the nervous sys- 

 tem of Synaptas as well as Hamann's ('83) and Cuenot's ('91) show that there is some 

 tendency to diversity in the nervous system, especially as regards sense-organs, among 

 the Synaptidae, and it also shows a considerable degree of adaptability to changing 

 conditions. Moreover, I have found in S. vivipara that the sense-organs are highly 

 developed, and there are additional nerves to the oesophagus, indicating modifications 

 to suit the mode of life. It seems from these facts, that too much stress must not 

 be placed on the opinion that the nervous system of Synapta digitata is primitive. 

 As regards degeneracy and the absence of anything in the mode of life to cause it in 

 SynaptaSj it seems that Semon has expressed an erroneous opinion of the causes of 

 degeneration. He says that we know of only three reasons for its occurrence, parasitic, 

 fixed, or subterranean life, and, since none of these are characteristic of Synaptas, they 

 cannot be degenerate. Had he given the matter more careful consideration he would 

 have seen how untenable his position is. Certainly no one will deny that the loss of the 

 power of flight in certain birds, as the famous New Zealand ground parrot, is degenera- 

 tion, yet they are neither parasitic, fixed, nor subterranean. Any change in the mode of 

 life, due to a change in environment, may result in degeneracy. The word has corne to 

 have a bad significance so that we think of it as indicating that the animal is on the 

 down-hill road, whereas it strictly means that the animal has lost some organ or group of 

 organs which its ancestors possessed and so has become less complex than they. Such a 

 loss must necessarily, however, always be a gain to the species involved, otherwise it 

 could never have come about. Now, it is entirely conceivable that in certain conditions 

 of life on the ocean bottom, in shallow water near shore, the loss of numerous ambulacral 

 appendages and the concentration of the water-vascular system in the circumoral ten- 

 tacles would be a distinct advantage to the animal. Such has certainly been the case 

 in Caudina (Gerould, '96), for instance, and it is probably true of all the Molpadiidae, 

 although in these cases it may have been brought about by subterranean life. But 

 Semon has by no means proved his point that the Synaptas are not, as a rule, subterranean. 

 Whatever may be the condition at Naples, both on the New England coast and in Jamaica 

 Synapta inkaerans and its allied forms are found normally buried deep in the sand, while 

 the large Synaptas, like S. lappa, are found under stones, which is practically a sub- 



