VOL. III.] Balanoglossiis. 193 



are probably in no wise connected, functionally, with the branchial 

 apparatus (certainly not in Balanoglossus); but since structurally 

 they are, and since we have no sure knowledge of what their 

 function is, we may well enumerate them along with the structures 

 in immediate connection with the branchial apparatus. 



Now, having spoken briefly of the parts in the organization of 

 Balanoglossus that do present strong resemblances to the corre- 

 sponding parts in Amphioxus, we must turn our attention to those 

 which do not. 



The proboscis, which is so characteristic of the animal, not only 

 cannot be compared with any structure in vertebrates, but the or- 

 gans which it contains, viz.: the " proboscis gland," the heart, and the 

 pore by which its cavity communicates with the exterior, are wholly 

 unrepresented in any vertebrate. Likewise none of the portions of 

 the abdomen lying behind the gill region can hardly be compared 

 with anything found in vertebrates. 



The structure of the body walls in the two animals is totally dif- 

 ferent. In Balanoglossus it is derived largely from the ectoderm, 

 the muscular portion derived from the mesoderm being compara- 

 tively weak and small, showing nothing of the muscle plates so well 

 developed in Amphioxus. Still it must be admitted that this con- 

 spicuous difference is rather secondary than fundamental since the 

 origin of the mesoblastic pouches presents considerable resemblance 

 in the two cases. 



On the whole, then, it seems to me that by a careful weighing of 

 all the evidence now at hand we are compelled to place this animal 

 in our classification nearer the vertebrates than to any other group 

 of animals (its comparison in several points with a remarkable 

 creature brought from the depths of the ocean by the Challenger 

 dredgings appears to be well founded. Unfortunately, however, 

 all our knowledge of this animal rests upon the adult structure of 

 a single species only and of a few individuals, even, of this one). 



Strongly beUving in the affinities of the larva of Balanoglossus 

 with the Echinoderm larva, Metschnikoff,^- in 18S1, attempted to fol- 

 low out the logical consequences of this belief and to reduce the struc- 

 ture of the adult Echinoderm and Balanoglossus to a common 

 fundamental type. The basal feature for this comparison is the 



^■' E. Metschnikoff. Ueber die systematische Stellung von Balanoglossus, Zool. 

 Anz., Bd. iv, 1881. 



