SPATANGINA. 93 



ambulacrum extending outward from the peristome, called a phyllode. The 

 interradial areas between the phyllodes may become swollen and conspicuous 

 and are then called bourrelets. The whole group of phyllodes and bourrelets 

 is called a floscelle. The floscelle is a taxonomic feature of great importance 

 in asternous forms, especially among the many fossil species. 



In all Recent Spatangina the periproct is outside and posterior to the 

 genito-ocular ring, which is thus a more or less compact group of plates, often 

 called the apical system. In many meridosternous forms the apical system 

 may be invaded by interradial plates and oculars I and V be quite separated 

 from the rest of the genito-ocular group; such a system is called disconnected; 

 or genitals 1 and 4 may push in and meet in the mid-line separating oculars I 

 and V from the anterior oculars, but remaining in contact with them; such 

 a system is elongate. In most spatangoids, however, it is compact and usually 

 occupies little space. Genital 5 is always wanting and its accompanying geni- 

 tal gland also. The genital gland and pore in plate 2 is frequently wanting 

 and occasionally in 3 also. In some species, at least, the genital pores appear 

 in genitals 1 and 4 before they do in 2 and 3 and in 3 before they do in 2, but 

 of how many species or genera this is true we have no idea. As a rule genital 

 2 contains the madreporic pores (indeed I know of no exception) and it is dis- 

 tinctly the largest of the genital plates. It extends backward in the mid-line 

 between oculars II and IV; if it does not separate oculars I and V also, the 

 apical system is said to be ethmophract but if, as is usually the case, it stretches 

 backward between I and V into interambulacrum 5, the system is said to be 

 ethmolytic. 



The periproct may lie on the dorsal surface in interambulacrum 5 (it is 

 always in this area) and in such cases is usually in a depression or groove. It 

 may be at the ambitus or it may be on the oral surface and occasionally close 

 to the mouth. It is always covered by a plate-bearing membrane; the plates 

 nearest the anus are smallest, those in the outermost series are commonly the 

 largest and may be very large and reduced to three in number. In most spatan- 

 goids the posterior end of the test is more or less truncate and the periproct is 

 located on that vertical or oblique surface. Below it there is often a group 

 of modified plates forming the subanal plastron; the tube-feet accompanying 

 some of the ambulacral plates entering into the composition of this plastron 

 are enlarged and more or less penicillate (like those near the mouth) ; the number 

 of ambulacral plates and big tube-feet in the subanal plastron is an important 

 taxonomic character, but in some species at least it increases with age or size. 



