DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 519 
appear to lead to the cavities enclosed by the radiating cells. 
These cords I propose to term the dorsal roots. They occur 
in B. minutus, Robinii, salmoneus, and Brooksil. 
Their homology will be discussed when the other morpho- 
logical questions arising out of these facts are treated of. 
The histology of the rest of the nervous system has been 
sufficiently described. 
The relations of the parts are explained by figs. 60, 64, 65, 
67, 73, &e. 
There are no special sense organs. 
As the “ dorsal roots” do not occur in B. Kowalevskii 
their development has not, unfortunately, been observed. 
The Hypoblastic Structures. 
The notochord has been described already, as also the 
mode by which the mouth comes to be anteriorly directed. 
The cavity into which the mouth leads is lined by very thick 
walls (figs. 90, 67, &c.), composed of long cells supported by 
some intracellular substance, probably the same as that of the 
notochord. In B. Kowaievskii it leads continuously into 
the branchial chamber, but in the other species, in which the 
branchial chamber is separated by longitudinal ridges (fig. 91), 
from the lower cavity of the branchial region (which thus has 
the well-known figure-of-8 shaped cavity). The anterior end 
of the branchial cavity comes to be almost enclosed in the 
pharyngeal cavity. As the result of this on either side the 
branchial cavity projects as two blind horns, which are en- 
closed in the pharyngeal cavity. 
The structure of the gill-slits has been sufficiently described 
by Kowalevsky, Agassiz, and Spengel. 
To these accounts there is little to add. The figures 84 and 
85 illustrate the mode by which their final structure is 
attained. It is practically impossible to follow their structure 
by means of transverse sections, but longitudinal sections and 
surface-views make them easily intelligible. Each gill-slit of 
B. Kowalevskii is U-shaped and surrounded by a skeletal 
secreted structure, as shown in fig. 85. In my last paper I 
