194 — Balanoglossus. | ZOF 
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 priort 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 ofanalogy 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 Miller. Ueber die Larven und die Metam 
Zweite Abhandlung, Abhandl. d. Akad. d. Wi 
orphose der Echinodermen, _ 
ss. zu Berlin, Juli, 1848. ee 
