CALCAREA — HETEROCOELA 



187 



still seem to assert their independence, the flagella not lashing 

 in concert/ but each in its own time and direction. 



Sub-Class II. Heterocoela. 



The Heterocoela present a series of forms of successive grades 

 of complexity, all derivable from 

 the Ascons, from which they differ 

 in having a discontinuous gastral 

 layer. The simplest Heterocoela 

 are included in the family Sycet- 

 tidae, of which the British repre- 

 sentative is Sycon (Fig. 79). In 

 Sycon numerous tubular flagellated 

 chambers are arranged radially 

 round a central cavity, the " para- 

 gaster," into which they open (Figs. 

 78, 79). The chambers, which are 

 here often called radial tubes, are 



Pig. 78. — Transverse section of the tody- wall 

 of Sycon carteri, showing articulate tiibar 

 skeleton, gastric ostia (a.^), tufts of oxeas at 

 the distal ends of the chambers (fl.ch), and 

 pores {p). (After Dendy.) 



FlO. 79.— Syoon coronatum. At a 

 a portion of the wall is removed, 

 exposing the paragaster and the 

 gastric ostia of the chambers 

 opening into it. 



close set, leaving more or less quadrangular tubular spaces, the 

 1 Vosmaer and Pekelharing, Verh. Ah. Amsterdam, (2) vi. 3, 1898, p. 1. 



