90 SYNOPTIC COLLECTION. 



It is probable that the ancestor of the class of Hydroids, 

 like that of the Porifera, was a fleshy animal without either 

 a horny or a stony skeleton, but under ordinary conditions 

 such a form would not be preserved. 



The skeleton of a primitive hydroid, Corynoides calicu- 

 laris Nich. (No. 99 ; PI. 100, drawing of the same en- 

 larged), is found as a fossil in the ancient geological 

 formations. It was tubular in form, chitinous in structure, 

 and striated on the outside as shown in the figure. The 

 body of the aclult, one may infer from the skeleton, was 

 tubular with an opening or mouth at one end raised, it may 

 be, on an oral cone, the base of which may or may not 

 have been surrounded by tentacles. This mouth probably 

 led into a hollow body cavity. The basal portion of the 

 tubular skeleton ended in two little spines, but there is no 

 indication in the fossils that the animal was attached, and 

 therefore the conclusion may be drawn that it was free- 

 moving both in youth and in adult life. 



Nothing is known of the development of this ancient 

 hydroid, but the simplicity of its structure as shown by its 

 skeleton leads to the natural supposition that the develop- 

 ment was primitive ; that is, without a metamorphosis of 

 any kind. 



The descendants of this single marine form may have 

 budded, and if the new zoons remained together a free- 

 moving colony would arise similar in some respects to 

 Graptolites (Nos. 101-104). 



According to Lapworth, 1 who has studied the develop- 

 ment of the Graptolitidae, the colonies arise from a " small, 

 pointed, triangular or rather dagger-like 'germ'," which 

 he calls the sicula. It may be that this youthful stage is 

 the representative of the single, ancestral Corynoides, al- 

 though this is not proved. In time a solid axis or virgu- 

 la develops in the outer wall of the sicula and often extends 

 beyond it at either end. A small bud usually appears at 



1 Geol. Mag., London, X, 1873. 



