502 



POLYZOA 



halves, exactly as takes place in the statoblasts, and a young 

 colony emerges. It is possible that the statoblasts may have 

 been evolved from a hibernaculum, 

 which was at first produced externally, 

 but has become modified in such a way 

 as to acquire an internal mode of origin. 1 

 The simplest known statoblast is 

 that of Fredericella (Fig. 251, A), which 

 differs from that of other Phylactolae- 

 mata in having no ring of air-cells. In 

 Plumatella, the statoblast (Fig. 251, B) 

 has a broad equatorial ring of air-cells, 

 which enable it to float at the surface of 

 the water on the decay of the parent 

 tubes. In some species, certain stato- 

 blasts which are produced in the adherent 

 parts of the colony remain attached to the 

 substratum. These " sessile statoblasts " 

 may have no trace of the ring of air- 

 cells ; but the fact that many sessile stato- 

 blasts have rudiments of this structure 

 suggests that they are a secondary modi- 

 fi^ion of the floating statoblast. In 



mains of part of a colony Lophopus (Fig. 251, C) the ring of air- 

 1 o a r 8 StbuVw 1 ""; cells is Vei 7 broad, and is pointed at each 

 2, zooecium. (From Krae- end; while in Cristatellci (Fig. 251, D) 

 and in Pectinatella the statoblast is 



circular, and possesses an armature of hooked spines. That of 

 Cristatella measures about '75 mm. in its greatest length. 



Kraepelin has suggested that the above order of increasing 

 complexity of the statoblasts corresponds with the order in 

 which the genera to which they respectively belong would be 

 placed, on the assumption that the Phylactolaemata have been 

 derived from the Ctenostomata. Thus, in Fredericella, the form 

 of the lophophore is circular, as in the Gymnolaemata. The 

 number of the tentacles is comparatively small (20-24). The 

 arborescent form of the colony resembles that of many Cteno- 

 stomes, and the zooecia are more or less cut off from one another 

 by incomplete septa. 



1 Cf. Kraepelin, Abh. Ver. Hamburg, x. 1887, No. 9, pp. 154 f. 



