194 SYNOPTIC COLLECTION. 



In the case of the Crinoids specialization not only 

 brought about greater regularity in the body plates but 

 arms were developed ; the ambulacra still served as food 

 grooves, through the holes of which numerous pointed 

 tentacles were put out. 



The digestive system in these ancient Echinoderms 

 was distinct from the body cavity and its two openings 

 mouth and anus were usually on the upper side. 



The Asteroidea, living to-day, pass through a transient 

 stage in their development when they are attached. In 

 becoming free they turn over so that the oral side is 

 below and the aboral above. In this favorable position 

 for obtaining food from the sea bottom by means of the 

 mouth, the pointed tentacles of the ambulacra develop 

 suckers and become locomotive organs or tube feet. It 

 is probable that further specialization causes the almost 

 useless ambulacra of the Ophiuroidea to become internal, 

 and the equally useless tube feet to take on reduced 

 characters. 



Besides the ambulacral plates of the typical Asteroidea 

 there are rows of interambulacral plates on the ventral 

 side, while the dorsal side is made of irregular plates. 

 The digestive system is complete, but in most cases the 

 anus opens opposite the mouth on the dorsal side. 



Water is admitted to the body cavity of the Asteroidea 

 through a sieve-like organ which connects with a series 

 of tubes that serve the double function of respiration and 

 locomotion. Greater concentration marked the organiza- 

 tion of the Echinoidea. The ambulacral and interambu- 

 lacral plates of the typical Asteroidea here reach an 

 extreme development, while the irregular plates have 

 almost wholly disappeared. 



All trace of a fixed stage is lost in the ontogeny of the 

 Echinoidea, so that the larvae are free from the start. 



The digestive and water-vascular systems are similar to 

 those of the typical Asteroidea, with the exception of 

 certain specializations such as the eating apparatus or 

 "teeth" and the reduced tube feet of many sea urchins. 



