Physiology of Trichodina pediculus. 413 
good one, one may trace the cilia to their very bases, with the per- 
fect confidence of not having seen amiss, and at the same time 
satisfy himself conclusively that they are unequivocally distinct 
from the veil which lies in front of them. There can be no 
hesitation, therefore, in pronouncing the veil and the vibratile 
row of cilia to be two distinct and separate systems, with no con- 
nexion whatever other than a close proximity of attachment to 
the basal margin of the body, and their similar duties in the 
process of locomotion ™~. 
The adherent organ (figs. 10, 17, h,.i, 2) is a complex appa- 
ratus, which altogether forms a thin circular disk, whose border 
(/') reaches to the margin of the base, or, in other words, to the 
inner edge or line of attachment (f?) of the velum (f). 
About one-third of the radius of the adherent organ, at the 
peripheral margin, is occupied by a sétriated annular membrane 
(2, 2, 1?, 08, 4, &), which is separable from the rest of the appa- 
ratus. It lies in front of the centrifugally projecting hooks (A), 
but closely pressed against them, and extends centripetally 
(to J) as far as their bases. This membrane is possessed of two 
sets of striz, which radiate from its inner to its outer margin. One 
set of strie occupy the anterior face (fig. 17, /' to /4), and are 
comparatively quite coarse (/?), and m number about ninety-six, 
i.e. four times the number of the hooks (/) of this organ. They 
le wide apart, and are arranged so uniformly that two traverse 
the interval between every two hooks, and two overlap every 
hook, where they run to the proximal margin (/°) of the mem- 
brane. In dead or dying specimens this membrane becomes 
folded or wrinkled (fig. 16, /') transversely, and then these striz 
(?) overlap each other and appear to fork more or less, or seem 
to be linear processes, divergent from the curved ends of the 
hooks (A)+. 
* See the note on the “adherent organ” at the end of this section, 
. 415. 
+ In the ‘ Proceedings of the Boston Society of Natural History’ for 
November 6, 1850, p. 354, Prof. L. Agassiz makes the following statement in 
regard to the relation of Trichodina to the Meduse, and especially in re- 
ference to these apparently forked, radiatimg striz, which remind one of 
the numerous radiating tubes of certain Hydroid Acalephz. He says :— 
“These parasites at times leave the Hydra and swim free, changing their 
form in a remarkable degree. In addition to the mternal ring, he was able 
to trace rays going from the hooks to the margin, divided into numerous 
branches, and also rays proceeding toward the centre from this ring ; the 
margin has a fringed undulating edge, under the tentacles. By feeding 
them with colours, he was able to see that the internal folds are the margin 
of a mouth, as in Rhacostoma; so that these parasites on Hydra are dimi- 
nutive Meduse. In the egg of Hydra, he had been able to trace all the 
forms from a segmental yolk to these parasites ; the freshwater Hydra is 
the Polypoid form of Meduse, while these parasites are the Medusoid 
form.” 
