570 INVERTEBRATE MORPHOLOGY. 



Development of the OpMuroidea. — ^Except in a few cases, such as Am- 

 pMura squamata, whose habits have already been referred to, the devel- 

 opment of the young Ophiuran takes place outside the body of the parent 

 in the surrounding water and a typical larval form occurs. This is known 

 as the Pluteus (Fig. 261) and in its general form resembles the Bipinnaria 

 of the Starfish. The ciliated band, however, does not divide into an 

 adoral and a postoral portion, but remains continuous, and the lateral 

 lobes become long armlike processes supported by a special skeleton, 

 formed of calcareous rods developed in their interior. The mode of devel- 

 opment of the young Ophiuran from this larva resembles closely that de-, 

 scribed for the Asteroidea. 



Class IV. Echinoidea. 



The Echinoidea present greater differences in shape than 

 are found in any of the other groups of Echinoderms, being 

 more or less spherical, oval, discoid, pentagonal, or heart- 

 shaped, but they are all characterized by the absence of arms, 

 by the calcareous plates being immovably united (except in a 

 few forms, such as Astlienosorna, where their edges overlap 

 and they are consequently movable) to form a firm test, and 

 by a great development of movable spines upon the plates, 

 whence the popular name of Sea-urchins usually applied to 

 members of the group. The test is covered by ciliated ecto- 

 derm, below wfhich is a plexus of nerve-fibres and ganglion- 

 cells which coordinate the movements of the spines, to whose 

 bases muscle-fibres are attached. 



The test presents certain variations in the different forms, 

 but there are also certain features which are to be considered 

 typical for the group. The apical system of plates is usually 

 well developed. A centrodorsal is present in the genus Sa- 

 lenia, but in all other recent forms it is replaced by a series of 

 small plates which constitute the periprod in the simpler 

 forms, since in these they surround the anus. These plates 

 are surrounded by a circle of five basals (Eig. 262, g), usually 

 termed genitals on account of the reproductive ducts opening 

 by a pore upon them. The five radials (o) are also repre- 

 sented, alternating with the basals ; and since the terminal 

 tentacle of the radial hydroccBl-cauals protrudes through a 

 pore situated upon them, and more especially since a pigment- 

 spot, supposed to be an eye, lies frequently at the base of 



