METAZOA ECHINODERMA. 161 



ASTEROIDEA. 



It is probable that Agelacrinus, belonging to the Agel 

 acrinidae, was a descendant of an ancestral form from 

 which the Asteroidea or starfishes of to-day arose. Using 

 a simple but clear illustration we may say that if the 

 hand should represent this trunk form, then the first fin- 

 ger would stand for the Agelacrinidae. Another finger 

 would represent the Asteroidea which began with the 

 same trunk but developed along another and quite inde- 

 pendent line. 



Since Agelacrinus comes nearer the probable ancestral 

 form than any other fossil or any living species, we begin 

 the study of the Asteroidea with this genus. Agelacrinus 

 (No. 290, A. rhenanus} was without a stem, but it was 

 attached by its dorsal or aboral side. The circular flat- 

 tened . body was covered by a great number of small 

 irregular plates which were imbricated or arranged like 

 shingles on a roof. These plates were perforated, and 

 usually the pores were in pairs. The mouth was in the 

 middle of the upper side and was covered by plates. 

 Radiating from the mouth were five ambulacra (No. 290) 

 also protected by covering plates. At the tip of each 

 ambulacrum there was a hole for the admission of water. 

 Each ambulacrum consisted of two rows of plates and 

 between these plates there were holes. 



The anus was situated in one of the areas outside of 

 the oral disc, and between two of the ambulacra ; it was 

 also covered by plates which were set into the body 

 plates. 



The irregularity of the body plates, the absence of- 

 arms, and the fact that the mouth, ambulacra, and anus 

 were concealed by plates, all remind one of both- the 

 Cystoids and the Blastoids. Zittel places this genus with 

 the Cystoidea, though it seems to have more points of 

 resemblance with the Asteroidea. 



