V. PHORONIDEA. VI. BRACIilOPODA 287 



air, causing the statoblast to float when it again comes into water. From the 

 statoblast a smaller polyzoon escapes which develops a new colony. The 

 statoblasts are adaptations to the conditions of fresh-water life and occur only 

 in the PhylactolEemata, where they arise as a sort of internal buds on the fu- 

 niculus, just before the degeneration of the polypids. 



Order I. Gymnolaemata (Stelmatapoda). 



Tentacles in a ring around mouth. Numerous species, almost exclusively 

 marine. CHILOSTOMATA, cystids can be closed by an operculum: Gemmel- 

 laria* Cellularia* Bugula* Fluslra* (fig. 274), £jc/!ura.- CYCLOSTOMATA, 

 tubular cystids without operculum. Crisia,* Tubulipora* Horn era* CTEN- 

 OSTOMATA, cystid is more calcareous, closed by a folded membrane. Alcy- 

 onidium* Vesicularia, Valkeria* marine; Pahidicella* (fresh-waterj. 



Order 11. Phylactolsemata (Lophopoda). 



Tentacles borne on a horseshoe-shaped lophophore extending on either side of 

 the mouth, the tentacles on its margins. All are fresh-water species. Pecti- 

 natella* Lophopus (tig. 275), PlumaleUa.* 



Class V. Phoronidea. 



The single genus Phoronis* was first called a chffitopod on account of its 

 worm-like body situated in a chitinous tube like many sedentary annelids. 

 Then it was placed in the Polyzoa, with which it is more nearly related. The 

 young, described as Aclinotrocha, is a modified trochophore with the mouth 

 overhung by a large hood and with the postoral band of cilia drawn out into a 

 series of fingers which become the tentacles of the adult; the anus is terminal. 

 At the metamorphosis this larva becomes doubled on itself, so that the alimentary 

 canal is U-shaped, the anus near the mouth, while the tentacles are borne on a 

 horseshoe-shaped basis around the mouth. 



Class VI. Brachiopoda. 



On account of the bivalve calcareous shells the Brachiopoda were long 

 regarded as molluscs, but later the fact that the valves are not paired 

 as in the lamellibranchs, but are dorsal and ventral, that the nervous 

 system, the excretory and reproductive organs, the body cavity, and the 

 development resemble those of the annelids rather than those of the 

 molluscs, led to their recognition as a distinct class allied to the former 

 group. 



The body has a greatly shortened long axis (fig. 276) and in conse- 

 quence a transversely oval visceral sac. In most a stalk {st) for attach- 

 ment arises from the posterior end. From the anterior side two folds, the 

 mantle lobes (p), extend forward, one ventral, the other dorsal, their free 

 edges bearing bristles. Each mantle secretes a calcareous shell. In a 

 few the dorsal and ventral shells are similar, but usually the ventral valve 

 (in Crania attached directly without the intervention of a stalk) is more 



