PYROSOMA THE FOUR PRIMARY ASCIDIOZOOIDS. 



407 



an oblique position and finally lie horizontally. This leads us to the 

 general changes of form which characterise the development of the 

 Ascidiozooid. The longitudinal axis of the adult Ascidiozooid (Fig. 

 201 C) seems to be marked by the branchial and atrial apertures (i) ; 

 in the bud, however, these two apertures do not lie at the ends 



FIG. 201. Three consecutive ontogenetic stages of two Ascidiozooids, side view, 

 diagrammatic (following SALENSKY). cf, atrium ; d, alimentary canal ; e, atrial 

 aperture ; es, endostyle ; i, branchial aperture ; ks, gill-clefts ; /?, nervous system ; 

 p, peribranchial cavity. 



of the longitudinal axis (Fig. 201 A). This shows that through the 

 changes brought about by growth during the course of development, 

 the longitudinal axis of the bud is replaced by a new one running 

 at right angles to it. The longitudinal axis of the young Ascidio- 

 zooid, the two poles of which are represented by trabeculae join- 

 ing one Ascidiozooid to another becomes, later, the transverse axis of 



