INTRODUCTION. XIX 



it would seem that these organs are not carried out in the 

 processes. Their number and the power which they pos- 

 sess of completely investing the zoophyte with their ex- 

 tensile threads would seem to show that they bear an im- 

 portant relation to the economy of the animal. They may 

 be concerned in the nutrition of the colony ; but without 

 further observations we can form no certain opinion 

 respecting their function*. 



In the genera Hydr actinia and Podocoryne, which ap- 

 proach the Siphonophora in complexity of structure, some 

 curious appendages occur in addition to the alimentary 

 polypite. We have first the spiral bodies, described under 

 Hydr actinia echinata (p. 24), which are developed on the 

 edge of the common base or crust, round the mouth of 

 the shell that supports the colony. They are remarkable 

 for the energy with which they uncoil and twist themselves 

 about when called into action. They usually form a 

 somewhat dense fringe round the mouth of the shell 

 (which is almost always tenanted by a Hermit-Crab), and 

 are roused from their state of quiescence by anything that 

 may irritate the surface of the coenosarc. As they are 

 well armed with thread-cells they may be regarded as 

 organs of offence, though it is difficult to understand how 

 they can act very effectively against the enemies of the 

 commonwealth. 



The other appendages referred to are long and extensile 

 filaments that spring from the surface of the cosnosarc 

 in certain portions of the colony, like the tentacles of 

 Velella, and which Dr. Wright regards as fishing-lines, 



* In the account of Hydr actinia echinata (p. 25) I have treated the ne- 

 matophorcs as offensive organs ; but since this passage was written I have 

 conic (o the cmiclusion that this is probably not their function. 



h 2 



