DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 523 
in living B. Robinii, and hence is probably due to reagents 
in B. minutus; as, however, I have never had an opportunity 
of seeing the latter in the fresh state this cannot be affirmed. 
In passing inwards from the outside to the centre of the 
proboscis the structures are thus arranged : 
1. Ectoderm.—Ciliated tailed cells. 
Glandular cells. 
Nerve-fibres as a layer. 
Basement membrane. 
2. Narrow tissue space crossed by ingoing fibres from ecto- 
derm, and by supporting fibres in all directions, together with 
a very few circular fibres (v. fig. 51). 
3. Tract densely filled with radial and longitudinal muscles 
(in B. Kowalevskii concentrically disposed in rings) and 
connective tissue. 
4. The tissue space into which the ceatral organs project. 
5. The central organs: 
(a) Proboscis gland with its sac. 
(6) Heart. 
(c) Notochord. 
The muscles of the collar body cavity in B. Kowalevskii 
are not gathered into bundles or definitely arranged, excepting 
those which are attached to the lateral rods of the axial skeleton 
(fig. 60). These large muscles are inserted into the back of 
the collar. The whole cavity between the pharynx and the 
skin, being originally second pair of mescblastic pouches, be- 
comes obliterated, being filled with muscles and connective tissue. 
In B. salmoneus, B. Robinii,and B. Brooksii this also 
occurs, but in B. salmoneus (fig. 106) the longitudinal 
muscles are grouped into bundles. These bundles form two 
series, the one on the somatic and the other on the splanchnic 
side, and in the narrower parts of the cavity the groups of the 
two series dovetail into each other (fig. 106), being each 
gathered around a connective tissue septum projecting into the 
cavity. 
These fibres in B. Robinii occasionally, after osmic acid, 
show a slight striping (fig. 94, ¢). 
