of the Ophiuridse and the Echinidse. 441 



the cavity is closed as in all the Echinodermata. At their first 

 appearance the feet are rounded at the extremity ; the disc is 

 formed at a somewhat later period. The spines, which soon ac- 

 quire a considerable length, contain a calcareous framework. 

 When the latter is perfectly developed, it forms an hexagonal 

 prism placed within the cylindrical skin of the spine, which con- 

 sists of a regular calcareous lattice-work terminating at the ex- 

 tremity in minute teeth. The horizontal arrangement of the axial 

 network of the spine is radiate, so that the extremity of the 

 spine viewed from above exhibits a star with six segments. 

 Before the framework of the spines is thus far developed, when it 

 first appears it has exactly the form of a candelabrum. The basis 

 of the framework of the spine is thus a star with six rays, from 

 the centre of which there arises a simple axis, which immediately 

 subdivides into other branches which subsequently re-unite. In 

 this manner a tubercle is formed which gives off some teeth 

 (Zacken) externally. The continued trunk again rises vertically 

 from the tubercle, six long arms proceeding from it ; these ascend 

 parallel, and give off teeth externally. The length of the developed 

 spines is so great, that it is about equal to the fourth part of the 

 whole disc of the animal. 



It is very enigmatical, that at first the tentacles or feet do not 

 appear in pairs, since these five odd tentacles do not occur in any 

 perfect Echinus or Echinoderm. I must leave it undecided, 

 whether the spinous disc under consideration corresponds to 

 the middle ventral portion with the framework of the teeth, as 

 it appears to do, or whether it is the dorsal part of the subse- 

 quent Echinus. If it were the dorsal portion, the five-cleft figure 

 would exhibit the five genital plates in the centre, and the seg- 

 ments from which the tentacles arose, between the above plates, 

 would correspond to the perforated plates, which M. Agassiz, with- 

 out sufficient ground, denominates the ocellar plates ; the centre 

 between the five original valves would then be considered as the 

 anus. At this period there is no aperture at this point, and the 

 spotted skin of the larva is continued over it. Moreover the shell 

 of the Echinus is at this time a delicate structure, the segments of 

 which growing into tentacles and spines cannot be considered as 

 the subsequent plates of the shell, but as the foundations of the 

 tentacles and spines. Moreover the foundations of the subsequent 

 arrangement of the tentacles in pairs may now be recognised, for 

 just anterior to those tentacles which are not in pairs, near the cen- 

 tre, there are two smaller, circular rudiments of the tentacles, lying 

 in pairs ; thus a circle of ten tentacles is produced ; and more to- 

 wards the circumference, rudiments of tentacula arranged in pairs 

 are visible. The disc itself, from which the tentacles and spines 

 arise, also contains its peculiar calcareous network, which is not 



