INTRODUCTION. V 



into a single sac or cell *, inclosing a mass of formative 

 material and certain rudimentary elements, out of which 

 a polypide is developed. These two constitute together 

 the primary term of the colony ; and by repeated bud- 

 dings, according to the pattern of the species, the com- 

 posite zoarium is built up. The colony is formed by 

 the indefinite repetition of the primitive zooids and their 

 modifications. 



The polypide consists essentially of a tube (alimentary 

 canal) bent upon itself so that its two orifices approxi- 

 mate, one of them (the oral) (Woodcut, fig. i. o) being 

 furnished with a number of ciliated tentacles (Wood- 

 cut, fig. i. t). In the tube or canal distinct regions are 

 recognizable — an oesophagus, stomach, and intestine 

 (fig. i. oes, st, ijit) : on the side of the oesophagus lying 

 nearest to the second of the two orifices (the anal) is 

 placed a nervous ganglion (fig. i. g). The alimentary 

 canal, thus constituted, is inclosed in a sac filled with 

 fluid (the zooecium) (fig. i. S), which possesses two open- 

 ings, corresponding with the two extremities of the tube : 

 through one of them the tentacles are expanded ; through 

 the other the rejectamenta are expelled (fig. i. a). In 

 most of the members of the class the upper or oral 

 extremity of the sac is soft and flexible, and is capable 

 of being inverted (or invaginated) and drawn within the 

 cavity when the polypide retreats into its cell, the in- 

 verted portion forming a sheath round the tentaclesf 



* •' La premiere zoecie, souche de la colonie nouvelle, n'est pas, comme 

 on I'a dit, engendrec par la larva. Cest la larve elle-meme qui se meta- 

 morphose comme la chenille se metamorphose en chrjsalide, c'est a dire 

 en conservant son individualite." — Joliet, Bryoz. d. cotes d. France, page 83. 



t This is not the case amongst the Enfoprocta (Fed kell in a and Loxosoma), 

 nor in Bhahdoplev.ra, the polypide of wliicli is quite unattached to its cell. 



