30 MUSCULAR SYSTEM. 



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 anterior and posterior parieto- 

 vaginal muscles in Pahdicella drawing after it as it descends that portion of the endocyst 

 which had been carried out during protrusion ; in the other genera, however, the anterior 

 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 preparation of the dissecting needle, for it is composed of a bundle of 

 elementary fibres, totally separate from one another through their entire course. The fibres 

 of the great retractor muscle are distinctly marked with transverse striae, a condition, however, 

 which is not at all times equally perceptible ; and some of our best observers have denied to 

 the Poh'zoa 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 the 

 fresh-water genera (PI. IX, fig. 6). In Pahdicella, I have seen this state beautifully marked 

 through the pellucid cell in the 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 

 which I have had an opportunity of examining it has, under favorable 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 strise become very obscure. When the broken extremity of a fibre 

 is examined, the fracture will be found to have occurred in a plane perpendicular to the axis 

 of the fibre, never presenting an uneven or lacerated surface, and a marked tendency to 

 separate into discs may be recognised in the detached and broken fibre. Indications of a 

 very delicate investing sarcolema may also be occasionally witnessed. When the fibre is in 

 an uncontracted state, it would seem to be perfectly cylindrical ; and the normal act of 

 contraction is so momentary that its condition during this act cannot be witnessed. When, 

 however, the living polypide is torn from its cell, the ruptured fibres which continue attached to 

 its body are thrown into a state of spasmodic contraction, and then it will be seen that they lose 

 their cylindricity and become ii-regularly swollen at intervals, while the whole fibre has mirch 

 increased in thickness : in this state we may also observe it obscurely striated. The swellings 

 here visible in the contracted fibre are quite different from the peculiar knots described by 

 Dr. A. Farre, in the muscles of the marine Polyzoa. Such knots do not exist in the 

 fresh- water species — at least I have never seen them — with the exception, perhaps, of certain 

 little swellings, which may be occasionally witnessed in the parietal muscles of Paludicella 

 and in the superior parieto-vaginal muscles of Plumatella. In Paludicella I have witnessed a 

 curious piienomenon presented by the muscular fibre. In this polyzoon the fibres of the 

 great retractor muscle, while lying relaxed in the bottom of the cell after the retraction of 

 the polypide, may frequently be seen to present a singular motion, impressing you with the 

 idea of a cluster of writhing worms. It is only in the great retractor muscles that I have 

 succeeded in detecting the striated condition of the fibre. 



It has been already shown (p. 12) that the fibres occurring in the endocyst (parietal 



