556 THE ANIMAL KINGDOM. 



anterior basal may become porous. The genital pore of the right an- 

 terior basal is sometimes lost in this family ; so also, but more rarely, 

 that of the left anterior if porous. Consequently as the posterior basal 

 and its pore are absent in all PetalosticJia, certain Spatangidae possess only 

 two genital pores (and glands) viz. those corresponding to the right and 

 left posterior interambulacra. The apical system retains the typical 

 disposition of its plates in Desmosticha and Clypeastroidea. It is then 

 termed compact. But among Petalosticha, the plates in most Andnchytidae 

 are arranged in two antero-posterior rows touching one another in the 

 middle line; and in the Collyritidae ( = Dysasteridae) the two radials cor- 

 responding to the two posterior ambulacra or bivium are separated by 

 a considerable distance from the other plates of the apical system to which 

 they are connected by two irregular rows of supernumerary plates. In the 

 former case the apical system is said to be elongated, in the latter dis- 

 jointed. 



The oral system of plates is represented in Palaeostorna mirabilis, one 

 of the Spatangidae, by five large interradial plates surrounding the mouth, 

 but it is doubtful if this system exists in other Urchins. 



The five ambulacral and interambulacral areae make up the corona 

 or test. The typical structure of the test is as follows : Each of the ten 

 areae consists of two rows of pentagonal plates, less numerous but larger in 

 the interambulacra. The pentagon is so disposed that an angle is turned 

 to the central line of union of the two rows in each area and a flat side to 

 the line of union between adjoining areae. The plates are firmly united 

 by their edges and the test is therefore resistent : a suture indicates the 

 line of union. Each plate is perforated by a double pore. Hence the 

 term pore-plate. The two pores give exit to two processes from the 

 ampulla within the test which unite and form a tube foot. The important 

 variations from the type structure are the following. Among Palaeo- 

 echinoidea, Bothriocidaris has but a single row of interambulacral plates, 

 whilst the number is increased in the remaining groups, as is the case 

 also in some genera in the ambulacra. The central rows in each area are 

 in these instances hexagonal. Among living Urchins Astropyga (Diadema- 

 tidae] and some species of Phormosoma (Echinothuridae) have an increased 

 number of interambulacral plates. There is an overlapping of the plates 

 in Palaeo-ecJiinidae, in Astropyga, and Echinothuridae, a group in which it 

 is well marked, and the test as a rule very flexible. An overlap is slightly 

 indicated also in Spatangidae, a group in which the two rows of plates 

 in the posterior interambulacrum not only are somewhat separate, but can 

 be approximated by muscles. In the same family the pore is single on the 

 majority of the plates except those at the apical and oral extremities. 

 The middle region of the expanded plates in the petaloid portion of the 

 ambulacra of Clypeastroidea may be marked by rows of fine pores in 



