Inter-relations of the Classes of tlie Echinodermata. 213 



Sub-stage i. Apelmatozoic. 



Class 3. ? " Some Cystideay 

 Class 4. ? Some Crinoidea. 

 Class 5. ? Some Blastoidea. 



Sub-stage ii. Pelmatozoic. 



Class 6. Crinoidea (s. s.). 

 Class 7. ''Cystideay 

 Class 8. Blastoidea (s. s.). 



2nd Sub-branch. Eleutuerozoa. 

 Division i. Zygopoda. 

 Class 9. Echinoidea. 



Division ii. Azygopoda (s. Stelleridea, s. em.). 



Class 10. Asteroidea. 

 Class 11. Ophiuroidea. 



Precision will be given to our ideas if concise definitions 

 of these various groups are given. 



The Echinodermata are Metazoa Coelomata in which 

 bilateral symmetry is early or altogether lost, but may be 

 secondarily acquired ; it is generally replaced by a quinque- 

 radial disposition of nearly all the parts. The integument 

 and some of the internal organs are strengthened by a crys- 

 talline deposit of carbonate of lime, mesodermal in origin, 

 plexiform in structure ; this may remain microscopic and 

 spicular, or part may form macroscopic rods or plates or give 

 rise to a continuous skeleton. A section of the coelom becomes 

 modified into a special system of sacs, canals, and tubes, 

 which form the water-vascular system, and have an ambu- 

 latory or respiratory function, or both. The sexes are gene- 

 rally separate, and development is rarely direct. 



1'hey are almost exclusively marine in habit. 



The Incaliculata are Echinodermata in which no system of 

 plates set alternately along and between the rays is developed 

 in the aboral region. 



The Anactinogonidiata are Echinodermata in which the 

 vascular and nervous, but not the digestive or reproductive, 

 systems exhibit quinqueradiate symmetry. 



The Caliculata are Echinodermata in which the skeleton is 

 always, in part at least, formed of plates, some of which are 

 set in rows, alternately radial and interradial, round a single 

 central plate. 



The Actinogonidiata are caliculate Echinodernuita in which 



