16 FRANCIS H. WELCH. 
tinuous with the longitudinal canals of the colony. From it 
a small branch appears to encircle each sucker, which after- 
wards passes down to merge into the main longitudinal canal 
of its side, so that a section through the head immediately 
below the suckers reveals the cut lumen of three tubes 
towards each lateral edge, while one through the neck some- 
what lower reveals only one (fig. 16, c), the smaller branches 
having very rapidly inosculated with the main trunk. 
Progressive development of the Zooids.—The so-called neck 
of the tapeworm is that part of the colony from which the 
production of new segments starts and their subsequent 
development. A transverse section of it (fig. 16) shows that 
the body structure with the inorganic granules irregularly 
arranged, the fibrous boundary of the visceral space with the 
contained granular albuminoid material, and the water-vas- 
cular canals, are all present in it as in the mature segment, 
the viscera only wanting. Immediately below it fine external 
transverse furrowings of the skin, closely approximated, 
foreshadow the segmental differentiation which is more clearly 
indicated internally by the delicate but decided transverse 
fibrous diaphragm. A longitudinal section through these 
early segments shows the transverse water-vascular canals as 
small and closely approximated channels, giving at first sight 
the idea of this portion of the colony being more freely sup- 
plied than elsewhere, yet not exceeding in fact the number of 
the component segments. ‘These transverse branches diverge 
more and more from each other as the segments lengthened 
from the head downwards, and, as well as the longitudinal 
canals, increase in calibre ; hence we may liken this system of 
vessels to a ladder, the sides corresponding to the main 
channels, the rounds to the transverse branches, and the 
larger terminal ends of the sides to the open lumen of the 
main channels in the lowermost segment, while we must con- 
nect the smaller terminal ends of the ladder together at the 
top by a thickish transverse branch to coincide with the canal 
between the pairs of suckers. At the 160th segment from 
the head the visceral space is more clearly defined by the 
darkness of the surrounding body structure from the 
amassing in it of the calcareous granules, and in the lower 
portion of it in the midline of the segment is a collection of 
delicate granular material freely imbibing colouring matter— 
first indication of the uterus. At the 200th segment the 
inflection of the body structures at the junction of the zooids 
is well pronounced, as also the fibrous diaphragm ; the lower 
extremity of the uterus is in the form of a globular granular 
mass close to the lower boundary of the segment, and curving 
