A PARASITIC OF THE TIGER SHARK. 
549 
cent one make up the principal mass of the neck as it: passes through 
the pseudoscolex, of which organ it is the axial center (figs. 17, 18, and 
22). The outer extremities of the massive parts of the cervical out- 
growths receive muscular fibers from the longitudinal body layer. 
These extend to the base of the fine outer folds, where they seem to 
blend with the cuticular limiting layer, in which the contractile power, 
which the outer folds undoubtedly possess, apparently in large meas- 
ure, resides (figs. 17&, also figs. 19 and 24). 
Near the base of the pseudoscolex the neck enlarges very rapidly 
(figs. 19, 20, and 24). The last cervical outgrowths to disappear are 
the two lateral ones. In fig. 20, d, d indicate the position of the last 
cervical process to disappear. The longitudinal muscle layer is here 
seen to approach the cuticular layer. In the section immediately pre- 
ceding this a few longitudinal fibers extend outward as radial fibers 
from these points. Figs. 19 aud 24 give views of longitudinal sec- 
tions of the head and pseudoscolex, but do not give a correct idea of 
the cervical processes in their entirety. The sections are made parallel 
to the lateral face of the body and pass through intervals between the 
cervical outgrowths. From the diagrammatic sketch, fig. 25, a some- 
what better idea may be obtained. Figs. 17 and 22, which are from 
camera-lucida drawings of transverse sections, give a correct represen- 
tation of these peculiar organs. 
The very powerful longitudinal muscles of the anterior part of the body 
and their continuation into the cervical processes and folds, where they 
are further supplemented by another powerful set of interlacing* trans- 
verse muscles, all indicate this to be a most efficient suctorial organ. 
This view is further substantiated by the fact that the structure of the 
external folds of the pseudoscolex is such that they may act as so many 
sucking disks. 
Here then is a very curious instance of the development of a special 
organ for a special purpose, instead of the modification of an organ 
already possessed ; or rather the development of an unusual part to 
perform a function which is normally performed by an organ which the 
animal already possesses. In kindred Cestods the organs of attachment 
are the bothria or cupping disks which are homologues of the sucking 
disks of the common Trnnia. These may be supplemented by various 
contrivances in the shape of secondary disks asjn Orygmatobothrium, by 
partitions dividing the bothria into loculi as in Evhetieibothrium, by hooks 
as in Galliobothriurn , by hooks and spines as in Echinobothrium , etc. 
The pseudoscolex of Thysanocephalum is probably a no more remarkable 
organ than the proboscis of Tetrarhynchus, but it appears to be unique 
in having become such a conspicuous and manifestly essential organ, 
while the true scolex and slender neck would be wholly inadequate of 
themselves to sustain the strain of supporting the adult body. 
That the head, though so miuute, still retains the supremacy, how- 
ever, is shown by the fact that the nervous system has its chief devel- 
