DETELOPMEITT OF THE PHTLACT0LiEMA.T0T7S POLTZOA. 497 



of tbelophopHore, and divides into branches on the intertentacular 

 membrane beneath the cellular layer ; but no closer connexion 

 of the nerves with the tentacles could be traced. Nitsche also 

 believes that he has seen a fine filament pass from the anterior 

 margin of the ganglion into the epistome, but cannot speak of 

 this with certainty. 



The central mass is stated by Hyatt to be contractile, and, as 

 a result, mutable in form. It is difficult to reconcile this character 

 with the properties of a true nervous centre, and one can scarcely 

 help believing that Hyatt's account of it rests on some deceptive 

 appearance. 



Statohlasts. — Hyatt describes the statoblasts of Fectinatella as 

 armed, like those of Cristatella, with spines ; but he has not 

 succeeded in detecting in the statoblasts of JPectinatella the 

 ciliated membranous envelope which in Cristatella surrounds 

 these bodies before their liberation. In Pectinatella they are 

 detached from the funiculus before the appearance of the spines, 

 and then lie loose in the body-cavity, where they remain until 

 the death of the polypide and the decay of the upper part of the 

 zooecium aff'ords them exit. They are then floated off", and 

 remain during the winter in a quiescent state, and often im- 

 bedded in ice. The young polyzoon, which on the approach of 

 spring protrudes from between the separating valves of the stato- 

 blast, has the whole of its free surface covered with vibratile 

 cilia. By the aid of these it enjoys for some time a free-swim- 

 ming existence, and finally disencumbers itself of the old valves 

 of the statoblast, loses its cilia, and becomes fixed. 



The peculiar statoblasts which are known to occur in several 

 freshwater species, and which, instead of being free, are always 

 found closely adherent to the walls of the zooecium, are regarded 

 by Hyatt as originating in these walls instead of being formed, 

 like the free statoblasts, in the funiculus. He also states that the 

 free statoblasts of Fredericella, though primarily formed, like those 

 of other genera, in the funiculus, become subsequently attached 

 to the walls of the zooecium, where they resemble in all respects 

 the true fixed statoblasts. 



Development. 



Development of the Bud. — Some very valuable contributions to 

 our knowledge of the development of the Phylactolasmata have 

 been made by Metschnikoff and by Nitsche. 



