376 
Tetrarhyncli us erinaceus 
The 'proboscides. These are the most characteristic organs'of the 
Cestode and enable one to identify it at a glance. They are differ¬ 
entiated very early in the development of the scolex, even before the 
bothridia have been formed, and the identification of the larva is always 
an easy matter when the proboscides can be seen. Usually they are 
invaginated in the larva, if so they can sometimes be extruded by 
gentle pressure on the cover glass while the worm is alive: but even if 
they are not extruded the large hooks can usually be seen. They are 
nearly always evaginated in the living worm and are extruded when 
the latter is killed by immersion in fresh water. There are four of 
these structures; and this number, with the presence of the notches 
and fossae on the bothridia, indicate that the primitive number of 
bothridia was also four. In a large Tetrarhynchus erinaceus the pro¬ 
boscides may be from 2 to 4 mm. in length and from 0'4 to O'o mm. 
in total diameter. Each proboscis consists of a sheath continuous from 
the tip of the organ to its insertion in the proboscis bulb. The sheath 
consists of two parts—proximal and distal. The posterior or proximal 
part is an integral part of the tissues of the scolex, is non-evaginable 
and is unarmed. The distal part is evaginable and is beset with hooks 
and spines on its internal surface. When it is invaginated it lies 
within the proximal part, but when it is evaginated the surfaces are 
reversed so that the hooks lie on its outer surface. 
The sheath of the proboscis is, in life, a clear, hyaline, tubular 
structure. The evaginable part consists of one uniform, structureless 
layer in which are inserted the hooks and spines. The part which lies 
within the scolex and is non-evaginable consists of two concentric 
laminae differing slightly in staining reaction. These layers are 
indistinctly separated from each other after fixation and the external 
one may exhibit an apparent vacuolisation. There are no muscle fibres 
on the sheath but here and there one may see scattered nuclei. The 
proboscides emerge from the scolex at the anterior and lateral margins 
of the bothridia, and at these points the tissues of the sheaths become 
continuous with the cuticle investing the body of the Cestode. 
The armature of the proboscis differs strikingly from that of other 
Tetrarhynchids with which I am acquainted and I propose to describe it 
in some detail since neither van Beneden (1858, p. 128, PI. XVII) nor 
Vaullegeard (1899, PI. XV) figures it very accurately^ PI. XX, figs. 6 and 
7 represent reverse and obverse views of the same proboscis (taken from 
a mature worm) and fig. 8 represents one view of a proboscis taken 
from a larva inhabiting the muscles of the Halibut. There may be 
