INTRODUCTION. XV 



passing animalcule or the floating food-particle towards 

 the central mouth. An examination of the structure of 

 the tentacle shows us on each side a layer of cells, fur- 

 nished with a large and brilliant nucleus ; and 

 from these the cilia originate as a simple Fig. v. 

 extension of the cell-wall. (Woodcut, fig. v.) 



Amongst the Gymnolamata the tentacles 

 are always disposed in a circle ; in the Phy- 

 lactolamata they range in a continuous series 

 round a crescentic lophophore. In the En- 

 toprocta the arrangement is somewhat ob- 

 scurely bilateral; whilst in the aberrant group 

 of the Pteruhranchia they are borne in dis- 

 continuous series on two lateral processes of 

 the lophophore. They are very variable in 

 number, ranging from eight (which seems to 

 be the minimum) to eighty in some of the 

 freshwater forms. Their movements are rai)id Ciliary cell 

 and energetic ; they can be bent hither and FredenceUa. 

 thither; they strike with vigorous percussive "Nucleus. 

 action ; they can be instantaneously coiled 

 into a spiral, and as suddenly unrolled. They assist 

 the ciliary currents, by their varied movements, in ob- 

 taining food, and help to keep off unsuitable or injurious 

 matter. 



But they are not only concerned with the direct nutri- 

 tion of the polypide ; they have also a distinct respiratory 

 function. The ciliated tentacles of the Polyzoa* are, no 



* " Gill-filaments " (Eay Laukester), iclentical with " the gills and labial 

 tentacles of the Lamellibvauchia and the spiral arms of the Brachiopods."' 

 See " Notes on Embryology and Classification," Quart. Journ. Micr. Soc. 

 new ser. no. 68, 1877, p. 423. 



