Vous. 11] - _ Balanoglossus. 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- 
gars 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 compara 
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 beliving in the affinities of the larva of Balanoglossus 
with the Echinoderm larva, Metschnikoff," in 1881, attempted to fol- 
_ lowout 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 
Cain Metschnikoff. Ueber die systematische scantitig: von Balanoglossus, Zool- 
_Anz., Bd. iv, 1881. 
