PLATE XV. 



Hydractinia echinata. 



Figs. 



1. Portion taken from near the margin of a female colony, magnified; a, a, a, hydranths in 



various states of contraction ; b, h, b, blastostyles laden with sporosacs ; c, c, c, spiral 

 zooids. 



2. A colony, natural size, spreading over the surface of a buccinum shell inhabited by a 



hermit crab. 



3. A blastostyle from a male colony, more magnified than fig. 1, laden with sporosacs. 



4. Transverse section of one of the chitinous spines from near the base; a, ectoderm investing 



the surface of the spine ; b, endoderm occupying its furrows and channels ; b', one of 

 the tubular cavities of the endoderm with its coloured lining ; c, one of the longitudinal 

 chitinous ridges of the spine. 



5. Transverse section of one of the chitinous spines from near the summit. 



6. A female sporosac seen in longitudinal section. 



7. Longitudinal section of one of the chitinous spines; a, superficial covering of ectoderm. 



b, b, endoderm with its included cavity occupying the superficial furrows of the spine . 



c, c, the chitinous walls of the spine showing successive layers of deposition ; d, d, longi- 

 tudinal chitinous ridges ; e, e, proximal portions of two hydranths developed from the 

 superficial ccenosarcal investment of the spine. 



