373 
THE ECHINUS, OR SEA-URCHIN. 
sea-lilies, which, with a number of extinct forms, constitute the 
class Echinodermata . 
All these forms agree together in having — 
1. Calcareous particles in the integument. 
2. A system of ambulacral vessels. 
3. A nervous system in the form of a ring round the gullet, 
with nerves radiating from it. 
4. Reproduction by means of a more or less roundabout pro- 
cess, i.e. by means of a more or less distinct secondary 
larva. 
They all differ from the lobster and the creatures like it, 
which belong to the annulose primary division of the animal 
kingdom, in that there are — 
1. No articulated limbs. 
2. No circulating system of true blood. 
3. No external skeleton. 
4. Not only no portal system, but no liver. 
5. No auditory organ or organ of smell. 
6. No chitinous envelope to the body. 
7. No known renal organ. 
8. No brain. 
They agree with such in that — 
1 . There is no solid internal structure separating the nervous 
centres from the alimentary canal. 
2. There are no jaws or visceral clefts. 
3. That in development the embryo does not present a 
longitudinal groove. 
4. That the anterior part of the alimentary canal is sur- 
rounded by the central part of the nervous system. 
The Echinodermata differ from the cuttle-fish and its allies, 
i.e. from the Mollusca , in that — 
1. The nervous system is not disposed in three pairs of 
ganglia. 
2. There is no liver. 
3. No limbs. 
4. No jaws. 
5. No circulating system of true blood. 
6. Never an external skeleton. 
7. No auditory organ or organ of smell. 
8. No known renal organ. 
9. No brain. 
They agree with such in that — 
1. There is no solid internal structure separating the nervous 
centres from the alimentary canal. 
