116 STAKFISH GALLERY. 



The Cidaridffi (489-495) are the only known living Endobran- 

 chiata. Other distinctive characters of the family are the large size 

 of the apical area (488, C) ; the auricles (488, D) to which the 

 Lantern of Aristotle is attached, are incomplete, or do not meet in 

 the middle line, and they are placed in the interradial areas. The 

 membrane (488, E) which bounds the mouth is covered by a large 

 number of both radial and interradial plates of small size. The 

 perforated radial plates remain separate, and do not form compound 

 plates as in most Ectobranchiata. 



The EcTOBEANCHiATE EcHiNOiDS (496-611) are divisible into 

 several groups, but all have external gills, radial auricles, and radial 

 plates only on the mouth membrane. Some have the apical area 

 large, and some possess the Organs of Stewart. 



The Aebaciid^ (498-502) differ from the Cidaridfe in being 

 ectobranchiate, and from the Echinothuriidse in having only five 

 pairs of buccal plates (decalepid series). 



The Saleniid^ ("^97) resemble the Cidarida3 and the Echino- 

 thuriid^ in having, as a rule, simple plates in the ambulacral regions. 

 In all other regular Echinoids several of the separate primary plates 

 are set in an arc, or fuse to form a compound secondary plate. 



The EcHiNOTHUEiiD^ (496) differ from all other Ectobran- 

 chiata in having more than five pairs of plates on the mouth mem- 

 brane, but they differ from the Cidaridse in not having interradial 

 plates carried on to that membrane. The Organs of Stewart are 

 sometimes large, sometimes lost. The test is thin and flexible, and 

 the paired plates overlap one another, so that the creature is able to 

 change in form. Eor evidence that the genera of this group are 

 extremely specialised and not primaeval forms, see Prof. Gregory, 

 Quart. Journal Geol. Soc, vol. liii. 



The EcHiNiD^ have the auricular arch complete, the apophysis 

 of the jaw is united in the middle line, the internal gill is generally 

 altogether lost, and three or more primary unite to form a secondary 

 plate. 



These may be strengthened by a process of interlacing, and 

 illustrations of sections through the test of a Temnopleurid (554) are 

 given to show the knobs and depressions, by means of which the 



