42 CATALOGUE OF THE BLASTOIDEA. 



CHAPTER IK. 



THE AMBULACRA. 



Those parts of the organization of a Blastoid which go by the general name of the 

 " ambulacra " form five radiating zones of variable width, that extend outwards 

 from the mouth in the centre of the summit, and are of very different lengths in 

 different genera. Whereas in Cryptoschisma (PI. V. figs. 23-26), Stcphanocrinus 

 (PI. XIX. figs. 8-10), and Cadaster trilobatus (PI. XIII. figs. 1, 4, 14) the ambulacra 

 are entirely limited to the ventral surface of the cup, those of Elceacrinus, Granato- 

 crinus, Mesdblastus, Schizoblastus, and of some Pentremites extend downwards along 

 the whole height of the calyx, and pass on to its dorsal surface (PI. II. figs. 1, 14, 

 32-35, 43, 46; PL V. figs. 14, 29; PL VI. figs. 12, 14, 15, 17, 23), sometimes only 

 terminating at the edge of the hollow in which the basals are lodged (PL VIII. 

 figs. 8, 18 ; PL IX. figs. 3, 13). Consequently, in such types as these, the ambulacra 

 have a very great resemblance to those of an Echinus, especially as they are con- 

 structed of closely-fitting limestone plates, and in the more typical Blastoids have a 

 series of pores along their margins (PL I. figs. 1-3 ; PL VIII. figs. 5, 21 ; PL IX. 

 figs. 5, 7, 14-10). It is not surprising, therefore, that Say 1 , the founder of the 

 group Blastoidea, and also of its originally single genus Pentremitcs, should have 

 been led to infer that these pores served for the transmission of respiratory tubes, and 

 that the Blastoidea " constitute the link between the Crinoidea and the Echinidea." 



The subsequent researches of Roemer, Rofe, Billings, and others have, however, 

 revealed the fact that the marginal pores of the ambulacra can have had no relation 

 to a median water-vessel, as is the case in the Urchins ; while four different generic 

 types have been discovered in which there are no pores at all along the margins of the 

 ambulacra. Xevertheless Say's doctrine has been recently revived by Hambach, whose 

 statements concerning it will be discussed further on. 



A. The Laxcet-plate and its Internal Relations. 



The various structures which enter into the composition of the ambulacra of a 

 1 '>l,is( oid will probably be most readily understood if we commence our study with the 

 type p;cnus Pentremitcs, which was so carefully investigated by Roemer. In like 

 manner we think it desirable to begin by describing the ambulacra as they appear in 



1 Journ. Acad. Nat. Sci. Philad. 18:25, vol. iv. pt. 2, pp. 292, !".•:!. 



