318 REPORT—1850. 
to a certain extent, for the inferior fibres of these muscles soon check its 
further progress. The remainder of the invaginated membrane, which in the 
retracted state constitutes the tentacular sheath, continues to be carried out- 
wards by the advancing polypide, the inferior parieto-vaginal muscles slowly 
relaxing to admit of it. These muscles, however, after a certain time refuse 
to suffer further relaxation, and thus afford a second check to the evagina- 
tion of the membrane. Thus we have two small permanent. invaginations 
existing after the completion of the protrusive act. One of these is placed 
within the other, and gives rise to the membranous cup which projects from 
the lips of the orifice in the exserted state of the polypide. This cup, there- 
fore, which may plainly be seen under a proper illumination to consist of a 
membrane doubled into itself, is nothing else than the imperfectly evaginated 
tentacular sheath. It may be seen during the act of protrusion in Pluma- 
tella and other genera; but in these it is.a mere temporary condition, being 
obliterated on the completion of the act. Mr. Hancock therefore appears to 
me to mistake the true import of this cup, when he maintains its homology with 
the crown of sete in Bowerbankia*. The true homologue of these sete is 
to be found, as has been already stated, in the cornecus ribs of the endocyst. 
When the protrusion of the polypide is complete, the last act in all the species 
is the display of the tentacula, which nad previously been all drawn together 
into a close cone or cylinder; and scarcely any more pleasing sight can be 
presented to the microscopic observer than the spreading out of the beautiful 
crown and the excitement of the vortices ‘in the surrounding fluid, by the 
countless cilia which instantly commence their untiring vibration on the 
sides of the tentacula. 
The mechanism of retraction is easily understood. Here the perigastrie 
fluid being no longer pressed upon by the contraction of the endocyst, the 
great retractor muscles act directly on the polypide and withdraw it into 
the cell, the superior and inferior parieto-vaginal muscles in Paludieella 
drawing after it as it descends that portion of the endocyst which had been 
carried out during protrusion ; in the other genera, however, the superior 
muscles would seem to take no part in this act. When the retraction is 
complete the sphincter closes the tentacular sheath, and the polypide rests 
secure in the recesses of the cell. 
The muscles of these animals are especially interesting in a physiological 
point of view, for they seem to present us with an example of true muscular 
tissue reduced to its simplest and essential form. A muscle may indeed 
here be viewed as a beautiful dissection far surpassing the most refined pre- 
paration of the dissecting knife, for it is composed of a bundle of elementary 
fibres totally separate from one another through their entire course. These 
fibres are distinctly marked with transverse strie, a condition, however, 
which is not at all times equally perceptible ; and some of our best observers 
have denied to the Polyzoa the existence of striated fibre. I have however, 
by repeated observations, satisfied myself of the striated condition of the fibre 
in the great retractor muscle in all the freshwater genera. In Paludicella, 1 
have seen this state beautifully marked through the pellucid cell in the ἡ 
whole extent of the retractor muscle while the fibres were on the stretch in 
the exserted condition of the polypide; and in all the other genera it has, 
under favourable circumstances of observation, been more or less visible. 
In order to witness. it in perfection the fibre must be on the stretch, for 
when this is torn from its attachments or lies relaxed on the bottom of the cell, 
the striz become very obscure. When the broken extremity of a fibre is 
* Loc. cit. 
—— νος δ“. 
