MUSCULAR SYSTEM. 20 



which Dr. A. Farre* attributes so large a share in the production of this act among the 

 marine Polyzoa, can at most raise tlie lophopliore and tentacula a very short distance, and 

 can exercise no exsertile influence on the inferior portion of the polypide, which, indeed, it 

 must rather tend to repel into the bottom of the cell ; while in all the fresh-water genera, 

 with the exception of Paludicella, the oesophagus, in the retracted state of the polypide, is 

 scarcely at all bent, so that here its agency in exsertion is at once out of the question. 



Let us suppose the polypide withdrawn into the recesses of the cell, and that hunger or 

 some other stimulus impresses on it a desire of protrusion. The endocyst now contracts on 

 the perigastric fluid, which, pressing on the polypide, forces it onwards towards the orifice ; 

 at the same time the vaginal sphincter relaxing, afi'ords to the cone of tentacula a free 

 passage through the tube of the inverted endocyst. 



The succeeding steps in the process take place somewhat difi^erently in the two great 

 groups. Li Plumatella and the other fresh-water phylactolsematous genera, as the polypide 

 continues to advance from the cell, the invaginated endocyst is gradually carried out with it 

 by a process of evagination, which proceeds up to a certain point, where it is stopped bv 

 tiie action of the posterior parieto-vaginal muscles, whicli, by straining upon the invaginated 

 membrane, had already afforded a fixed line, on which it rolled outwards during eversion, 

 This line constitutes the extreme limit of eversion, and that portion of the invaginated 

 endocyst which lies between it and the mouth of tlie cell remains permanently invaginated. 

 In Paludicella the process is somewhat more complicated ; here the relaxation of the anterior 

 parieto-vaginal muscles permits the eversion of the endocyst, but only to a limited extent, for 

 the posterior fibres of these muscles soon check its further progress, keeping one portion per- 

 manently invaginated, and afl"ording a fixed point on which the remainder may roll outwards. 

 This second portion of the invaginated membrane, which in the retracted state constitutes the 

 tentacular sheath, continues to be carried outwards by the advancing polypide, the posterior 

 parieto-vaginal muscles slowly relaxing to admit of it. These muscles, however, after a 

 certain time refuse to suffer further relaxation, and thus afi^ord a second check to the 

 evagination of the membrane. Thus we have two small permanent invaginations existing after 

 the completion of the protrusive act (PI. X, fig. 4). 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, therefore, which may plainly be seen under a proper illumi- 

 nation to consist of a membrane doubled into itself, is nothing else than the imperfectly 

 evaginated tentacular sheath. It may be witnessed during the act of protrusion in Plumatella 

 and other genera ; but in these it is a mere temporary condition, being obliterated on the com- 

 pletion of the act. When the protrusion of the polypide is completed, the last act in all the 

 species is the display of the tentacula, which had previously been all drawn together into a 

 close cone or cylinder; and scarcely any more pleasing sight can be presented to the micro- 

 scopic observer than the spreading out of the beautiful crown and the excitement of the 

 vortices in the surrounding fluid, by the countless cilia wliicli instantly commence their 

 untiring vibration on the sides of the tentacula. 



The mechanism of retraction is easily understood. Here the perigastric fluid being no 



* Observations on tlie Minute Structure of some of the higher forms of Polypi, ' Pliilosopliical 

 Transactions,' 1837. 



