DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. ol? 
the skin of the base of the proboscis. In the larva with four 
gill-slits (fig. 99, P. rg.) it is already well marked. Concen- 
trations are formed in the line of the gill-slits (figs. 72a and 
104), and slight fibrous anastomosing tracts run irregularly, 
following the line of the wrinkles from both the dorsal and 
ventral cords. These wrinkles taper towards both the cords 
and are permanent, being, in fact, limiting lines between 
patches of glandular cells. 
Now, all these tracts of fibres are thickened as they approach 
the central nervous system, and dwindle peripherally. If this 
diminution were due to the continual separating of efferent 
fibres from the cords it would reasonably be anticipated that it 
would be greatest in the case of those parts of the body which 
lie behind the collar (1. e. behind the central nervous system) ; 
for these cords have almost the whcle body to supply, but, 
on the contrary, it is the nervous sheath of the proboscis 
which presents the greatest concentration, and this continually 
thickens on approaching the collar, though the proboscis is 
conical and its base is towards the collar. This may be 
taken to show that this sheath of nerve-fibre is afferent, and 
is continually increasing in thickness owing to the incoming of 
sensory fibres from the ectoderm cells lying above it. Its 
sudden increase on the proboscis stalk is due to the sudden 
tapering of the base. This feature is particularly well seen in 
B. minutus. On any other hypothesis it would seem un- 
likely that this great deposition of nerve-fibre should occur in 
a region which is generally covered up by the anterior folds of 
the collar. 
The Central Nervous System.—The changes occurring 
in this structure in B. Kowalevskii after its separation 
consist in an increase in size and in histological differentiation. 
As the result of these changes its anterior end comes to have 
the structure shown in fig. 60. Among the cells lining the 
anterior end of the lumen are always some few gland-cells, 
The cellular part of this cord is continuous, of course, with 
the cellular part of the skin, and the fibrous part or white 
matter, as we may call it, with the fibrous layer of the skin, 
