172 



L WER IN VER TEBRA TES. 



In 3Iellita the test becomes lai-ge and heavy, and the edges present deep cuts 

 opposite to the ambulacra. In Encope the niassiveness of the test increases, reaching 

 its fullest development in E. grandis, a native of the west coast of Mexico and of 

 the Gulf of California. It is hard to believe that the mass of calcareous material 

 forming the test of this sea-urchin ever contained a living animal. The edges of the 

 test are half as thick as the thickest part, which is at the anus. There is a huge 

 luinile between the posterior ambulacra, beside five cuts opposite the ambulacra. 



Order III. — PETALOSTICHA. 



These sea-urchins, more commonly known as irregular sea-urchins or Spatangoids, 

 have no dental apparatus; the test is variable in form, though usually more or less 



Fig. 151. — Pirriiiujisis hjrifera. 



ellijjtical ; and the anal system is placed between the two posterior ambulacra (bivium). 

 Certain parts of the test and spines are greatly specialized ; and the radiate form is 

 accompanied with an evident bilaterality. 



Neither the oral nor the anal apertures are in the centre of the test, the former 

 being displaced anteriorly, and situated beneath the odd anterior ambulacrum, while 

 the latter is situated beneath or between the petals of the bivium. The ambulacra in 

 this order vary greatly, but are always petaloid in character upon the ujjper surface, 

 the series of pores not being continuous around the edge of the test to the under 

 surface. The anterior ]ietal or ambulacrum often becomes more or less abortive, 

 so that there are only lour jielals visible above ; while in other cases it is much 



enlarged. 



