VIII 



CALCAREA HETEROCOELA 



IS/ 



still seem to assert their independence, the flagella not lashing 

 in concert, 1 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 



Fig. 78. Transverse section of the body-wall 

 of Sycon carteri, showing articulate tubar 

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

 the distal ends of the chambers (flxh), and 

 pores (p). (After Dendy.) 



Fig. 79. Sycon 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. Ak. Amsterdam, (2) vi. 3, 1898, p. 1. 



