METAZOA ECHINODERMA. 187 



IRREGULAR SEA URCHINS. SPATANGOIDS. 



One of the ancestral forms of the more specialized 

 division of the irregular sea urchins commonly called 

 Spatangoids is Pyri?ia subsphaeroidalis d'Orb. (No. 347), 

 in which the test is high and dome-shaped and the ambu- 

 lacra are arranged in the primitive fashion of vertical 

 rows from mouth to apex. The mouth is nearly central 

 in this genus, while the anus is on the dorsal side of the 

 posterior part. 



The young forms of this group which are living to-day 

 are similar to the young of the regular urchins and of the 

 Clypeastroids. In fact the starting point of these groups 

 is the same, as shown by A. Agassiz. 1 



The Spatangoids have at first the vertical row of pores 

 running from mouth to apex, few tubercles of large size, 

 spines of disproportionate length and size, and a simple 

 lipless mouth. The adults, however, carry specialization 

 much farther than any other members of the class. 



Beginning with the more generalized forms we find 

 Echinoneus semihmaris Lam. (No. 348), is dome-shaped 

 with the ambulacra in vertical rows and no petals formed 

 throughout life. The mouth is near the center and as in 

 many Spatangoids is without teeth, while the anus in this 

 genus is between the mouth and posterior end. Four 

 genital and five ocular pores are seen in the abactinal 

 area. 



In addition to the ordinary tubercles Echinoneus has 

 others that have the appearance of glass and carry no 

 spines. 



Hybodypus caudatus Wright (No. 349), is a small 

 somewhat flattened urchin with the three anterior ambu- 

 lacra separated from the two posterior ones. Here 

 the mouth has moved a little towards the anterior side 

 while the anus is in a depression on the dorsal side. 



1 Proc. Amer. Assoc. Adv. Sci., XXIX, 1880, p. 397. 



