82 HUBERT LYMAN CLARK ON 



a step further and normally never develops at all, but disappears altogether, which is 

 especially interesting as the mid-ventral radius is the first to develop its nerve and oto- 

 cysts, and so seems to be the leader in modifications. The manner of life has also caused 

 a modification of the tentacles in a way which we may consider as a degeneration from 

 other Synaptas. Semon ('87) describes calcareous rods in the tentacles of all the Medi- 

 terranean Synaptas, and these appear abundantly, as we have seen, in the young stages of 

 S. vivipara, but in the adult they seem to have almost entirely disappeared, the tentacles 

 and digits being very delicate and flexible and containing no calcareous deposits, except 

 some miliary granules. The changes in the larva due to the retention and development 

 of the ova in the body-cavity of the mother, such as closing of the blastopore and absence 

 of any true metamorphosis, must also be taken into consideration. For these reasons, we 

 must consider S. vivipara as a highly specialized Synapta, but in its water-vascular system 

 it has degenerated a step further than S. digltata, although it is neither "parasitic, fixed, 

 nor subterranean " in its manner of life. 



It is very clear from the examination of the literature on the subject that the study 

 of any one form or class of echinodenns is entirely insufficient to fit one to determine on 

 a theory of the phylogeny of the group. Notable examples of this may be seen in the 

 speculations of Semon ('88), Biitschli ('92), and MacBride ('96). The facts added to 

 our knowledge of echinoderm embryology by all these writers are of real value, but their 

 hypotheses are for the most part of little importance. The same may be said of any 

 attempt to determine the entire course of echinoderm evolution by the study of palaeon- 

 tology alone, a notable example of which has recently appeared by no less an authority 

 than Haeckel ('96). The only author who has carried on original investigations on all 

 the classes of echinodenns and has formulated his views on the phylogeny of the group is 

 Bury ('80 and '95), and I cannot conclude this paper without calling attention to the 

 support which my observations give to him, on the questions involved in the development 

 of the Synaptidae. Regarding all the points on which he lays particular stress, I have 

 confirmed his work completely or in part. The adradial position of the water-tube, the 

 rudimentary left anterior enterocoel, and the growth of the left body-cavity around the 

 oesophagus are all very clearly marked in the development of Synapta vivipara. The 

 only point on which I could not entirely confirm his views was on the formation of the 

 mesentery of the stone-canal from the left coelom entirely, and on this point what evi- 

 dence I did obtain indicates the correctness of his position. 



