92 HAWAIIAN AND OTHER PACIFIC ECHINI. 



terminal, but on the other hand it is sometimes central and very rarely back 

 of the center. It is usually more or less sunken, occasionally to an extraor- 

 dinary degree but on the other hand it is often nearly or quite flush with 

 the oral surface. The peristomial membrane is usually more or less calcified 

 with numerous irregular small plates: these, however, are siri generis and not 

 homologous with any plates of the test. Occasionally the membrane is quite 

 naked, while in Palaeostoma, the other extreme is reached, the mouth being 

 completely concealed beneath five triangular, presumably movable plates. 



When the peristome is anterior, the interradius directly posterior to it 

 (interambulacrum 5) is commonly more or less modified to form a sort of 

 creeping surface called the sternum. If the interambulacra are all alike or ap- 

 proximately so. the animal is said to be astemous. In asternous forms, each 

 interambulacrum retains, at the edge of the peristome, the single primordial 

 interambulacral plate followed at once by the pair of plates of series 2. In the 

 forms with a sternum, however, one or more of the interambulacra may have 

 the primordial interambulacral plate followed by a second unpaired plate. 

 The origin and homology of this second plate is as yet unknown. If present 

 at all it will be found in interambulacrum 5. and it may also occur in all the 

 other interambulacra or only in 1 and 4. In most Spatangina. however, the 

 primordial interambulacral plates are followed (even in 5] by the pair of plates 

 of series 2. Those species which have the primordial plate of 5 followed by a 

 single plate are called meridosternaus while those in which it is followed by a 

 pair (and they are usually much enlarged so that they are the largest plates of 

 the test) are called amphisternous. Further we may call the interambulacra 

 which have the primordial plate followed by a single plate, meridoplaeoms, 

 while those in which it is followed by two plates are amphiplacous. In all 

 forms in which the sternum is at all well developed, the primordial plate of 

 interambulacrum 5, has a special form (generally more or less T-shape and its 

 anterior margin forms the posterior boundary or lip labium' of the peristome: 

 the plate itself is called the labrum. The relative length of the labrum as com- 

 pared with adjoining ambulacral plates is of considerable taxonomic importance. 

 rule, the sternum is covered with primary tubercles and spines, but in some 

 specialized genera, more or less of the anterior half may be quite bare. The 

 tube-feet about the mouth are generally enlarged and developed into curious 

 tactile brushes. These enlarged feet are naturally associated with enlarged 

 pores and peripodia. When these enlarged pores are numerous, and espe- 

 cially if they are at all crowded, they form a somewhat petal-like group in each 



