340 NATURAL SCIENCE [May 



and are separate from one another, ' inacUinate ' as it is termed. 

 Modification of this takes place in two directions, nearly but not 

 quite the same, and lioth described as the incorporation of the lower 

 or proximal brachials in the dorsal cup so as to support or surround 

 the viscera. 



In the one case this incorporation takes place by the intercala- 

 tion of interbrachial pieces, and these and the * fixed ' or incorporated 

 brachials are rigidly united by the firm mode of union known as 

 ' close suture.' Pari passu with this incorporation of brachials in 

 the cup there necessarily goes an incorporation of ambulacrals in the 

 tegmen and their fixation by means of ' interambulacrals,' which are 

 merely supplementary plates like the interbrachials. A further 

 effect of this rigid fixing of the elements of the theca is that the 

 ambulacrals of the tegmen cease to open, so that the food-grooves 

 instead of being open gutters become closed tubes and pass beneath 

 the orals to the underlying mouth. The process may even be carried 

 further : the interambulacrals increase in size, and the ambulacrals 

 may be squeezed down between them, so that all except a few of 

 the larger ones are invisible from the upper surface of the tegmen ; 

 these larger ones become prominent and are called ' radial dome- 

 plates.' Thus arises a form in which all the ambulacral structures, 

 including the various extensions of the nervous, generative, and 

 other systems of the body are absolutely subtegminal, and appear to 

 be covered over by a rigid dome or vault of solid plates. So much 

 is this the case that Wachsmuth, to whom the earliest complete 

 account of the structure is due, supposed that the vault was actually 

 a fresh formation that had somehow or other grown right across the 

 normal ventral surface. That the vault is not really so anomalous 

 a structure, but merely a modified tegmen, has only been proved 

 within the last seven years by Wachsmuth and Springer, and is the 

 view maintained in their present work. 



In the second case incorporation of the lower brachials may be 

 either by lateral union with those of adjacent rays, or by supple- 

 mentary interbrachials, or by a finely plated integument ; the differ- 

 ence is that the union is not rigid, but all plates above the radials 

 retain some power of yielding or flexibility, even if they have not 

 always the faculty of active motion. Of necessity correlated with 

 the flexibility of the cup is the flexibility of the tegmen, which here 

 gradually increases by decrease in size of the plates, or even by their 

 complete disappearance ; even the orals and ambulacrals have 

 atrophied in some of the later representatives of this group. 

 Another difference is the persistence of mouth and food-grooves as 

 suprategminal, i.e. open on the ventral surface. 



In accordance with these three types of structure it is possible 

 to group the crinoids into three Orders. This is done by Wachsmuth 



