194 Balanoglossus. [zok 



water system in the two groups, the proboscis of Balanoglossus being 

 supposed to represent a single ambulacral tentacle of an Echino- 

 derm. This is certainly a most ingenious speculation and one that 

 must be admitted to be not wholly without plausibility, especially as 

 regards this particular structure. The resemblance in other points 

 of structure is very obscure, and it should be remembered 

 that similarity between groups in several fundamental points of 

 structure increases the probability of homology between these 

 structures — and so of genetic relationship between the animals pos- 

 sessing them — many times beyond the number of points of resem- 

 blance. For example, we can see no a pr-iori reason, either 

 physiological or morphological, why a water system should not ex- 

 ist in correlation with several styles of animal organization. Con- 

 sequently, when we find an animal possessing it that in other re- 

 spects resembles other animals that possess it very obscurely if 

 at all, the probability that the system is homologous in the two 

 instances is not very great, it seems to me; at any rate there is 

 great room for the possibility of analogy merely, i. e., that the structure 

 has had an independent origin in the two cases. When, how- 

 ever, there is an essential agreement in several points of organiza- 

 tion, as we have seen to be the case between Balanoglossus and 

 vertebrates, the probabilities of mere analogy or independent origin 

 are many times less. 



Developmentally Balanoglossus presents some most interesting 

 chapters in phyologenitic history — interesting both on account of 

 the parts of them that we can understand, and of those that we 

 cannot, as yet satisfactorily interpret. One of the most strikingly 

 interesting things in this history is that the different species 

 do not tell the same story, that they do not all present the same ped- 

 igree, and this is true, notwithstanding the fact that they are all so 

 closely related that no one has ever pretended to claim more than 

 specific differences between them. 



So far as is known all the species excepting one pass through a 

 very distinct and quite prolonged larval stage. This one — an 

 American form — develops without any larval stage. The 

 larva was discovered by the distinguished German zoologist, 

 Johannes Miiller,^^ in 1848. He was at this time studying the em- 



'^ Johannes Milller. Ueber die Larven und die Metamorphose der Echinodermen, 

 Zweite AbhandUing, Abhandl. d. Akad. d. Wiss. zu Berlin, Juli, 184S. 



