24 



kind. The tentacles and mesenteries have become very 

 numerous in most of the forms existing at the present day. 



The ancestral Madreporaria, on the other hand, must have 

 acquired the property of producing a calcareous skeleton 

 which was not confined to the axis of the colony, as in the 

 Alcyonaria and the Antipatharia, but was developed in the 

 mesoderm, not only of the colony, but of the polypes also, 

 so as to form a skeleton for each member of the colony. In 

 most of the groups of Madreporaria now existing, repro- 

 duction by budding takes place to a great extent, large and 

 complicated colonies being the result. 



Returning now to the main stem of the tree, we find that 

 above Gastrea it passes up into that intensely interesting 

 region which is the origin of the various groups of lower 

 Vermes. From this it may be traced upwards through the 

 starting points of all the great groups of higher Metazoa, 

 the Mollusca and the Chordata, the Crustacea and the 

 Tracheata, to its termination in the Polychaeta — the highest 

 Vermes. From Gastrea to the base of Polychaeta then, the 

 main axis of the tree may be considered as consisting of a 

 series of ancestral worm-like forms extending from the most 

 primitive, the first modification of a Gastrula, up to the 

 immediate progenitors of the higher Annelides. 



What the changes were by means of which the Gastrea 

 passed into one of the ancestral lower Vermes is difficult to 

 determine. The body probably became elongated, and a 

 mesoderm was developed between the two primary cell 

 layers ; but whether the elongation took place along the 

 antero-posterior axis of the Gastrea, so that the aperture 

 remained as a terminal mouth-opening, or at right angles to 

 that axis, so as to convert the aperture into an elongated slit 

 placed upon one surface, is a disputed point. Balfour* and 



* Comp. EmbryoL, vol. ii, p. 308. 



