186 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Blastoids. 



In the blastoids we find the entire body enclosed within a capsule formed by 

 plates all of which are comparable to plates arising in the dorsal body wall of the 

 crinoids. The radials of the blastoids grow forward on either side so that the 

 ambulacrals are developed within a furrow formed between their two branches. No 

 perisomic surface is exposed. 



While in the blastoids the development of the visceral mass and of the external 

 skeleton is equally balanced so that the latter always completely encloses the former, 

 in the crinoids quite different conditions obtain. At first the development is 

 similar; but in the crinoids the development of the calyx plates is abruptly arrested 

 while the visceral mass continues its growth. 



It is necessary for the ambulacral plates, represented by the third and follow- 

 ing brachials in the crinoids, always to maintain the same relationship with the 

 ventral ambulacral structures. In the blastoids the ambulacrals grow over and 

 cover in the ventral ambulacral structures, new plates being continually formed 

 near the ventral apex. In the crinoids they are turned outward and form a support 

 over the ventral surface of which the ambulacral structures run. 



But in the crinoids the visceral mass grows so fast that the ambulacral plates 

 or brachials, necessarily permanently attached to the edge of the ventral disk, 

 become widely separated from the radials by an area of naked perisome. This 

 naked perisome, belonging to the primarily skeleton forming dorsal surface, sup- 

 ports calcareous plates which form connecting bands between the radials and the 

 proximal ambulacrals. 



The presence of this series of plates intermediate in character and in position 

 between the radials and the ambulacrals (which eventually come to form the division 

 series and first two brachials) and the turning outward of the latter are the essential 

 differences between the blastoids and the crinoids. 



In the urchins the external portion of the test is formed entirely by the small 

 apical system and plates comparable to the division series and first two brachials 

 of the crinoids, with the radials represented as 10 ambulacral plates around the 

 peristome. True ambulacrals, comparable to the ambulacrals of the blastoids and 

 to the arm ossicles of the crinoids from the third brachial outward, are represented 

 by the auricles and by the complicated dental pyramids, while the so-called ambu- 

 lacrals are not true ambulacrals at all, but are plates developed in the intermediate 

 perisomic area between the plates of the apical system and the base of the true 

 ambulacrals, which correspond to the plates proximal to the radials in the crinoids. 



It is because of the fact that the so called ambulacrals of the urchins are not 

 true ambulacrals of the type seen hi the blastoids at all, but merely pseudo-arnbu- 

 lacrals developed originally as space fillers, that in many types they are multi- 

 columnar. True ambulacrals are from the very nature of their origin invariable 

 biserial or secondarily monoserial. 



The blastoids are essentially imperfect, or, more properly speaking, too perfect 

 crinoids, and in a sense they are remotely intermediate between the crinoids and 

 the echinoids. They possess the characteristic structures of crinoids, yet their 



