24 MUSCULAR SYSTEM. 



contraction of the endocyst, there can, however, now be no doubt. Van Beneden* tells us that 

 lie has seen these cilia, not only on the walls of the perigastric space, but on the external 

 surface of the alimentary canal. I cannot, however, confirm their existence in the latter 

 situation ; indeed, my own observations are entirely opposed to their presence on the alimen- 

 tary canal ; and I cannot help thinking that this statement of Van Beneden is connected with 

 some error of observation. I have, however, most distinctly seen them on the inner surface 

 of the upper part of the tentacular sheath in certain species during the exserted state of the 

 polypide (PI. IV, fig. 4) ; on other parts of the endocyst I have not succeeded in detecting 

 them by direct observation ; but the peculiar acceleration which the motion of the circulating 

 corpuscles experiences when these approach the walls of the perigastric space, plainly 

 indicate the presence of vibratile cilia in this situation. 



(4.) Muscular System. 



The muscular system is highly developed ; we shall first consider it in the phylactolaema- 

 tous Polyzoa, and afterwards attend to its disposition in Paludicella. 



a. PliyladolcEmata. 



In all these the disposition of the muscles is exceedingly similar ; eight distinct sets may 

 be considered as more or less positively demonstrated. 



(1.) Retractor Muscles of the Polypide. 



These, which are the largest and most powerful muscles of the animal, consist of two 

 fasciculi (PI. IV, fig. 4 ; V, fig. 5 ; IX, fig. 7, n), which arise far back from the inner surface of 

 the endocyst, and thence pass forwards, one along each side of the alimentary tract, to be 

 inserted into the upper part and sides of the oesophagus. A few accessory fasciculi (PI. V, 

 fig. 5, o) may also be generally seen arising near the origin of the former, and inserted into 

 the sides of the stomach. The action of the retractor muscles is very obvious ; having a com- 

 paratively fixed point of attachment near the bottom of the cell, they retract the whole 

 alimentary canal with the tentacular crown, so as to place them in a state of security in the 

 interior of the coenoecium. 



* Quelques Observations sur les Poh'pes d'eau douce, toe, cit. 



