METAZOA ECHINODERMA. 163 



Gradually the pentagonal form tended to give way in 

 Palaeozoic time to the stellate form illustrated by the 

 ancient starfish Palaeaster (No. 292, model). The plates 

 of the ahoral side of the central disc are small and indefi- 

 nite, but those of the arms are more or less regular 

 (No. 292, specimen on the left). The anus is found on 

 the aboral side, though the model does not show its posi- 

 tion. The mouth is uncovered, but is surrounded by 

 five plates. The ambulacra are also uncovered and con- 

 sist of two rows of alternating plates (No. 292, specimen 

 on the right) ; on the outer edge of each plate there is an 

 opening for a tentacle (No. 292). Thus there are two 

 straight rows of holes extending through each ambulacral 

 groove. 



On either side of the ambulacral plates there is a 

 row of prominent interambulacral (adambulacral, Zittel) 

 plates (No. 292 b), and outside of these are large margi- 

 nal plates. 



Respiration was effected in these early forms as in 

 modern species by means of a water system consisting of 

 a sieve or madreporic body, which in ancient forms was 

 on the ventral side (Zittel), and of radiating tubes. 



The primitive forms are also represented by A stropecten 

 variabilis Liitk. (No. 293), in which the marginal plates 

 are conspicuous. Here the arms are more tapering than 

 in Ctenodiscus, but the tentacles are still pointed and 

 remain in two rows. 



In Hippasteria phrygiana Ag. (No. 294), the ambu- 

 lacral plates are opposite and not alternating ; the tenta- 

 cles are in two rows, but instead of being pointed they 

 are provided with sucking discs and are, therefore, no 

 longer true tentacles but rather locomotive organs or 

 " tube feet." These organs are seen in the preparation 

 (No. 295) which is a dissection to show the water-vascu- 

 lar system after injection with blue coloring fluid. This 

 system will be described more at length under the com- 

 mon starfish, Asterias (see pp. 165-169). The ovaries 



