METAZOA ECHINODERMA. 189 



A giant among its kindred is Metalia pectoralis A. 

 A|j. (No. 355). Some of its spines are peculiarly modi- 

 fied, being so extremely long and delicate that they are 

 rarely preserved. These are attached to small areas on 

 each side of the dorsal median line and within the fasci- 

 cle. They are capable of lying flat upon the corona, as 

 seen in No. 355. On other parts of the body the spines 

 vary in length and in some places they are distinctly 

 curved. The mouth is near the forward end and the 

 anus in the large blunt posterior extremity. The marked 

 sexual difference in size is shown by No. 355 ; the female 

 is much larger than the male, while the latter is rarely 

 found. The corona denuded of spines is a beautiful 

 object (No. 356). 



Another large, robust urchin is Meoma ventricosa Liitk. 

 (No. 357, ventral side). Here similar spines cover 

 the whole surface. The mouth (No. 357) has a large 

 lip and the anus is at the posterior end of the body. The 

 sunken petaloid ambulacra above are conspicuous. 



Among the most specialized of the irregular urchins is 

 Moira atropos Ag. (No. 358). Here different parts of 

 the test are made of variously shaped plates. The mouth 

 is far forward, and the anus is at the upper side of the 

 blunt posterior end. There appear to be deep slits on 

 the upper side and on looking more closely the ambu- 

 lacral pores are seen at the bottom of four slits. The 

 fifth one, which is more like a groove than a slit, extends 

 forward and turning downward reaches the mouth. The 

 young are carried in these sunken ambulacra after much 

 the same fashion as in Hemiaster. The variation in the 

 shape and size of the interambulacral plates is great. 



An extreme of specialization is reached when one 

 examines Amphidetus cordalus Ag. (No. 359). The short 

 silky spines (No. 359, c) conceal the ambulacra and 

 interambulacra which are most curiously modified. In- 

 stead of simple vertical rows or the usual rosette of petals 

 there is here a star-like arrangement of the ambulacra 



