146 SYNOPTIC COLLECTION. 



lutely luminous with the light it throws upon the phylo- 

 genetic history of its group. So far from being mislead- 

 ing, such an ontogeny is the normal and inevitable result 

 of the operation of the law of acceleration in development 

 by which adult characters are inherited earlier and earlier 

 in the life of the individual, until it may be they appear in 

 the embryo only, and finally disappear altogether. 



According to A. Agassiz 1 the Cystoids and Blastoids 

 represent among fossil Echinoderms the nearest approach 

 we have yet discovered to the imaginary prototype of the 

 subkingdom of spiny-skinned animals. 



The characters possessed by the living Echinoderma are 

 such that they can be explained satisfactorily only by 

 supposing that these animals are the descendants of 

 attached forms. It may be possible, as already suggested, 

 that the ancestors of the attached forms, living in some 

 remote pre-Cambrian age, were free-swimming, and that 

 these free-swimming adults are represented, with few or 

 many modifications, by the larvae of existing Echino- 

 derms, molluscs, and worms. Be this as it may, the fact 

 is pretty well established that our present free-moving 

 Echinoderms are directly descended from fixed or sta- 

 tionary ancestors. 



One of the simplest Cystoids is Aristocystis (PI. 251, 

 figs, i, 2, A. bohemicus Hkl.). Here we have a body pro- 

 tected by many calcareous plates placed together irregu- 

 larly. It was attached by its base (fig. 2), which was 

 surrounded by more regular plates ; as yet no stem had 

 developed. The slit-like mouth was on the upper side 

 (fig. 3, m), and at one side was the anus (fig. 3, a) 

 closed by a pyramid of plates. Between these two open- 

 ings there was a pore, now thought to be the genital pore 

 (fig. 3, g). Near the mouth there was still another open- 

 ing supposed to be for respiration and called a hydropore 

 (fig. 3, h). In this genus there were no specialized areas 



1 Proc. Amer. Assoc. Adv. Sci., XXIX, 1880, p. 411. 



