PLATE X. 



Strongylocentrotus Drobachiensia. 



1. Specimen, nat. size, with tentacles fully expanded from abactinal side. 



2. l.e""- in diam. including spines, wliere spines have assumed the shape of adult, 



with pedicollaria and anal plate. 



3. Specimen somewhat younger, having only five tentacles, from actinal side. 



4. The same seen fi-om abactinal side. 



5, 6. Expanded tentacles of adult, magnified. 



7, 8. Pedicellariaj of the abactinal part of test. 



9, 10. Pedicellariae round the actinostome. 



11-13. Pedicellariae of abactinal part of test in different stages of growth. 



14, 15. Pedicellariai of abactiual part of test, seen from above. 



