HYDKACTINIA ECHINATA. 41 



out, that these, and the " snake-like " bodies 

 may have analogy with the nematophores 

 found in the Plumulariice, or to certain 

 organs in Ophiodes. 



The nature of the chitinous crust is not 

 very clear. The plan, however, appears to be 

 a system of horizontal horny tubes more or 

 less irregular, compressed closely against 

 one another, and opening into each other, thus 

 forming a sheet which covers the surface of 

 the whelk shell. The exposed surface of the 

 tubes, or crust, is perforated similarly to 

 perforated zinc, and allows the coenosarc 

 which fills up the interior of the tubes, also to 

 overspread the outer surface of the crust. 

 From this surface layer of coenosarc the naked 

 polypites emanate. 



This species is named EcMnata, from the 

 spiny character of the crust, the spines 

 evidently serving to protect the polypites 

 from friction against objects in the many close 

 corners into which the Hermit crab takes 

 them. Beneath these spines, or pinnacles, the 

 polypites crouch and remain unharmed. 



