14 STRUCTURE OF THE CYSTIDE^. 



existence until the Carboniferous period, and the graptolites disap- 

 peared early in the Upper Silurian. With the exception then of 

 the graptolites, the Cystideae were the first race that became extinct. 



II. The General Form and External Skeleton of the Cyslidea.. 



In form the Cystideae were either globular, oval, pyriform, conical, 

 or sub-cylindrical, and their dimensions seldom more than one inch 

 and a-half or two inches in length and breadth. They were protected 

 by an external skeleton composed of flat polygonal calcareous plates, 

 which were so accurately fitted together that they enclosed, with the 

 exception of the arms and column, the whole of the animal almost as 

 completely as an egg is contained in its shell. In some of the species 

 the plates were neither limited as to their number, nor arranged 

 according to any definite order, and in these, as the body increased 

 in size, the corresponding enlargement of the skeleton was effected 

 both by the growth of the older plates and the introduction of new 

 ones between them. In many species the number of the plates 

 and plan of arrangement remained constant throughout the life of 

 the animal, the shell being enlarged by the continual growth of 

 the original plates and without the addition of new ones. In others, 

 such as the species of the genus Pkurocystites, both of these modes of 

 increase prevailed, the dorsal side having the number and arrangement 

 definite and the ventral indefinite. The growth of the individual 

 plates appears to have been by the assimilation of fresh particles of 

 matter throughout the whole mass, instead of by additions to the edges. 



III. The Mouth, Ainbulacral Orifice and Anus. 



In the Cystideae we find two and in some species three principal 

 apertures through which the more important functions of the animal 

 economy were exercised. These are : — 



1. The mouth, — A large orifice situated on one side, usually about 

 the middle of the body, but sometimes near either the base or the 

 apex. In many species it was provided with a valvular apparatus, 

 by which it was opened or closed; in others no such provision 

 existed, or at least it has not been preserved in the fossil state. 

 It is quite probable that in most of the species this orifice subserved 

 the double function of a buccal and an anal aperture. 



2. The amhulacral orifice. — This opening is always situated in or 

 near the centre of the upper part of the body, and in the central 



