14 FRANCIS H. WELCH. 
face ; this is limited to the longitudinal canals at the free end 
of the lowermost zooid of the colony. 
Having now described the anatomical structures present 
in an adult segment, it becomes necessary to detail the link- 
ing of the zooids together, the head, and the modifica- 
tions of each zooid in the process of development of the 
colony. 
Bond of connection between the zooids—The lower end of 
each segment is slightly expanded, while the upper end of the 
succeeding one is contracted and invaginated in the one 
‘above by an oblique infolding of the body wall from below 
upwards and continuous ar ound the segment. ‘This infolding 
implicates the skin, circular and transverse muscular fimmese 5 
some of the longitudinal muscular bands terminate in the pit 
left- by the infolded structures, others curve around it, 
while the more internal pass from one segment to another. 
From the invaginated portion a thin fibrous band passes 
transversely through the body structure mapped out by the 
paucity of calcareous nodules on each side of it, and with a 
similar one within the visceral space forms a diaphragm 
separating one segment from the other, penetrated only by 
the longitudinal “water-vascular canals, some of the longi- 
tudinal muscular bands, and the visceral boundary. Hence 
the only components which can be regarded as special to each 
segment are, male and female generative systems, transverse 
water-vascular canal; all other structures are continuous and 
common to the entire series. 
Head of the Colony.—As before mentioned, this is some- 
what square-shaped and larger than the neck, towards which 
it tapers. It partakes of the character of all other segments 
in being wider than it is thick, but in a much less ratio. On 
its free end there is a cross furrow, with an inflexion of the 
skin at the centre into a pit, and on the elevations between 
the furrows and the free edge of the head are the four suckers 
seated. Immediately below the suckers are regular trans- 
verse folds of the cuticle—the earliest differentiation of the 
one segment from the other, and commonly, though not 
necessary or constant, a large quantity of dark granular pig- 
ment is collected in the tissues between the suckers. ‘The 
sucker may be defined to be a globular inflexion of the cuticle 
with muscular adjuncts for a special function—that of 
anchoring the colony. A section through it (fig. 15) reveals 
as follows:—The epidermis and corium of the general 
cuticle are continuous through it, forming the inner lining of 
the hollow globe (fig. 15, d), only that the corium is slightly 
thickened, firmer and more elastic, and its deeper layer 
