Fig. 185.—Half of a transverse section through a proglottis of Tænia crassicollis. Cu., cuticle; Ex.v., external excretory vessel, to the right of which there is the smaller internal one; T., testicular vesicles; L.m., longitudinal muscles (outer and inner); M.f., lateral nerve with the two accessory nerves; Sc.c., subcuticular matrix cells; Sm.f., submedian nerve; Tr.m., transverse muscles; Ut., the uterus, and the middle of the entire transverse section. 44/1.
The mass of parenchyma bounded by the transverse muscles is termed the MEDULLARY layer, while the mass lying outside them is termed the CORTICAL LAYER.
It was known long ago that the myoblasts adhere to the dorso-ventral fibres as thickenings, but it is only recently that large star-shaped cells (fig. 184), separated from but connected with them by processes, have been recognized as the myoblasts of other fibres (Blochmann, Zernecke).
Within the scolex the direction and course of the muscular layers change.
Fig. 186.—Dipylidium caninum: from the cat. In the upper figure the rostellum is retracted, in the lower protruded, a, sucker; b, hooks of rostellum; B, enlarged hook; c, apical aperture on scolex; d, longitudinal muscles; e, circular muscles. (After Benham.)
The SUCKERS are parts of the musculature, locally transformed, with a powerful development of the dorso-ventral muscles, now become radial fibres.
The ROSTELLUM of the armed Tæniæ, like the proboscis of the Rhynchobothriidæ, also belongs to the same category of organs.
In the simplest form, the rostellum, or top of the head (as in Dipylidium caninum), appears as a hollow oval sac, the anterior part of which, projecting beyond the upper surface of the head, carries several rows of hooks (fig. 186). The entire internal space of the sac is occupied by an elastic, slightly fibrous mass, while the anterior half of the surface of the rostellum is covered by longitudinal fibres and the posterior half by circular fibres. On contraction of the latter the entire mass is protruded through the apical aperture, the surface of the rostellum becomes more arched, and the position of the hooks is, in consequence, altered. The rostellum of the large-hooked Tæniidæ, which inhabit the intestine of man and beasts of prey, is of a far more complicated structure, for, in addition to the somewhat lens-shaped rostellum carrying the hooks on its outer surface, there are secondary muscles grouped in a cup-like manner (fig. 187). Every change in the curvature of the surface of the rostellum induces an alteration in the position of the hooks. In the hookless Tæniidæ the muscular system of the rostellum is altered in a very different manner; in a few forms a typical sucker appears in its place.