288 



COMPARATIVE ANATOMY 



CHAP. 



Order 2. Paractinopoda. 



Only some of the outer appendages of the water vascular system arise from the 

 radial canals, the rest from the circular canal, and the only form taken by them is 

 that of tentacles round the mouth. 



Family 1. Synaptidse. 

 Tube-feet wanting. Mouth terminal. Body cylindrical, more or less elongated 

 and vermiform. 10-27 feathered or digitate tentacles. Stone 

 canals occasionally numerous. Retractor muscles sometimes 

 present. Respiratory trees and Cuvier's organs wanting. Sexual 

 glands often hermaphrodite. Synapta (Fig. 229), Ohirodota, 

 Myriutrochus. ^ 





CLASS II. Echinoidea (Sea-urchins). 



The body of these Echinoderms is covered by a usually firm 

 but sometimes flexible test, which contains the Offilomic cavity 

 and the viscera. The test varies in shape, from spherical to a 

 form which is flatly compressed in the direction of the principal 

 axis. It consists of numerous pentagonal or hexagonal closely 

 contiguous plates, which, arranged in meridional rows, form five 

 ambulacral and five interambulacral areas. It is covered by the 

 outer layer of the integument, and carries spines articulating 

 with it. At the apical pole there is a system of plates, consisting 

 of five basal plates, five radials, and the anal plate. The mouth 

 is usually in the middle of the oral surface, less frequently shifted 

 towards the edge in what is called the anterior direction. An 

 anus is always present, either at the apical pole or at some part 

 of the posterior interambulacral area. The apertures of the 

 madreporite lie in the apical system, generally in one of the basal 

 plates ; they are connected not only with the stone canal but 

 with the so-called dorsal organ. The ambulacral vascular system 

 has outer appendages developed as tube-feet and gills. Mouth 

 with or without teeth. In the former case a complicated 

 masticatory apparatus is developed within the test for the move- 

 ment of the teeth ; the muscles moving this apparatus are 

 attached to a perignathous apophysial ring developed at the 

 edge of the oral aperture of the test {i.e. round the peristome). 

 Sexually separate or hermaphrodite. The genital ducts open 

 externally through pores in the basal plates or outside these 

 latter. Development direct (with care of the brood), or with 

 metamorphosis (free-swimming larvte). 



Sub-Class 1. PalEeechinoidea. 



Either only one row or more than two rows of plates in each 



dleltata (original) interambulacral area. Two or more meridional rows of plates in 



each ambulacral area. Plates of the test do or do not imbricate. 



Oral aperture of the test (with peristome) in the middle of the oral surface. Jaws 



^ The arrangement of the classes and families of the Holothurioidea by Ludwig in 

 Bronn's Klassen wad Ordinniijen des Thierreichs, 1892, is here followed. 



