ECHINODERMS. 93 



spines, as in the Sea-urchin and Star-fish, in others they are either 

 comparatively insignificant or completely suppressed, as in the 

 Crinoids and Ophiurans. Such are the three regions or systems 

 which by their greater or less development introduce an almost 

 infinite variety of combinations into this highest class of Radi- 

 ates. It may not be amiss before proceeding further to compare 

 the five orders with reference to this point, and see which of 

 these three systems has the preponderance in each one. 



Taking the orders in their rank and beginning with the lowest, 

 we find in the Crinoids that the dorsal system preponderates, 

 being composed of highly complicated plates, and developed to 

 such a degree as to form in many instances a stem by which the 

 animal is attached to the ground, while the ambulacral system is 

 limited to a comparatively small area, and the interambulacral 

 system is wanting. The order of Crinoids has diminished so 

 much in modern geological times that we must consult its 

 fossil forms in order to understand fully the peculiar adaptation 

 of the. Echinoderm plan in this group. 



In the Ophiurans, the dorsal system is still large, and though 

 it no longer stretches out to form a stem, it folds over on the un- 

 der side of the animal so as to enclose entirely the ambulacral 

 system, forming a kind of shield for the arms. Here also the in- 

 terambulacral system is wanting. 



In the Star-fishes the dorsal system encroaches less upon the 

 structure of the animal. The back and oral side here correspond 

 exactly in size, and though the flat leathery upper surface of 

 the animal, covered with spines, serves as a protection to the 

 delicate ambulacral suckers which find their way between the 

 rows of small plates along the under side of the arms, yet it does 

 not enfold them as in the Ophiurans. On the contrary, in the 

 Star-fishes the ambulacral rows are protected on either side by a 

 row of the so-called interambulacral plates, through which no 

 suckers pass. 



In the Sea-urchin, the dorsal system is contracted to a mini- 

 mum, forming a small area on the top of the animal, the rows of 

 interambulacral plates which are separated and lie on either side 

 of the ambulacra in the Star-fish being united in the Sea-urchin, 

 and both the ambulacral and the interambulacral svstems bent 



