242 



ANNULOIDA. 



(" Heart-urchins ") the test is oval, oblong, or commonly 

 heart-shaped; the ambulacra (fig. 137, A) are petaloid, the 

 anterior one unpaired, usually lodged in a groove or " sul- 

 cus," and thus rendering the skeleton bilaterally symmetrical. 



^..*;*;ii -.:^:-'* 3' :- -"V^ S ^ 



iiil ; i^ 



Fig. 137. A, Upper surface of Micraster coranguimim, of the natural size Cretaceous ; B, 

 Under surface of Ananchytes ovata, of the natural size Cretaceous. (After Edward Forbes.) 



The anus is posterior and supra-marginal. The mouth is 

 inferior, eccentric, and toothless. The tubercles are small, 

 and support hair-like spines, but there are larger, crenulated, 

 and perforated tubercles for supporting larger spines. As a 

 rule, bands of microscopic tubercles known as "fascicles" 



(fig. 138) are present, and oc- 

 cupy different positions in dif- 

 ferent genera. Sometimes the 

 " fasciole " surrounds the am- 

 bulacral rosette, when it is said 

 to be " peripetalous ; " some- 

 times it is " internal," sur- 

 h Mt d rounding the unpaired ambu- 

 lacrum; sometimes it surrounds 

 the sides, and is said to be 

 " lateral ; " at other times it runs round the test, and is 

 termed "marginal;" and lastly, it may be limited to the base 

 of the anal aperture, when it is termed " sub-anal." Of the 



hand figure shows the " fasciole " cutting 

 the ambulacral rosette. 



