ANATOMY OF TEHENIA MEDIOCANELLATA. 15 
strongly pigmented (e), the latter feature rendering the 
sucker a very conspicuous object in the naked-eye anatomy of 
the parasite. Next to the deep corium layer are two special 
series of muscular fibres, one radiating, the other circular ; 
the former pass from the corium to a globular fibrous layer 
(fig. 15, f) which follows the contour of the sucker at a dis- 
tance from it of =); in., forming a point d’appui for the 
radiating proper layer on one side, and for the insertion of 
muscular bands external to it on the other; the circular fibres 
completely encircle the sucker in concentric lines within the 
fibrous envelope. ‘The diameter of the interior of the sucker 
averages ;!, in., of its circular orifice ;'; in., that of its fibrous 
capsule ;5 in. Occasionally dark pigment is also collected 
on the exterior of the capsule, mapping it out conspicuously 
from the surrounding textures, and on its exterior also (ex- 
ternal as compared to the sucker, but actually internal as 
compared to the head generally) the longitudinal muscular 
bands common to the whole colony spread out fan-shaped, and 
are inserted into it (fig. 15, 4). Hence these external bands 
retain the fibrous capsule in a fixed position for the special 
muscles to act from, the radiating bands. expand the area of 
the hollow globe and so induce the function, the circular 
bands contract it, and so put a stop to the sucking action, 
ending in freeing the colony from the anchoring ground—the 
mucous lining of the small intestine; the sucker being fixed, 
the fibrous capsule would equally form a point for the longi- 
tudinal muscles of the colony to act from, and to move the 
entire series of zooids. ‘The direction of the axis of each 
sucker is towards the corresponding angle of a square. ‘T'ra- 
versing the centre of the head from the neck is a horizontal 
fibrous layer which divides the suckers into pairs, into what 
may be termed a dorsal and ventral pair, the latter cor- 
responding to the flat surface of the zooid indicated as ventral 
by the position of the genital ducts to the water-vascular 
system and the junction of vagina with uterus. This fibrous 
diaphragm is the termination within the head segment of the 
visceral boundary-wall common to the colony ; when nearing 
the neck the walls approximate somewhat from the absence 
of viscera to distend them (fig. 16, 4); in the head they are 
in contact, and terminate on the inner surface of the skin. 
Between the pairs of suckers, just beneath the skin, and 
corresponding in direction to the fibrous stratum (fig. 14, f 
to g), is the central water-vascular canal of the head, the 
transverse or connecting branch of the cephalic segment. 
Its diameter is much larger comparatively ; it curves down 
at each end between the lateral suckers, and becomes con- 
