150 



THE STORY OF LIFE IN THE SEAS. 



This association is, from the Crab's point of 

 view, a more advantageous one than that with 

 the Sea-anemone, for it does away with the 

 necessity of any changes of shell, the Crab 

 and the Sponge growing up together. The 

 history of the companionship is probably as 

 follows: A small Hermit-crab takes for its 

 shelter a small Gastropod shell, and upon this 

 shell a Sponge larva settles, grows and spreads, 

 until it surrounds the whole of it except the 



hole from which the 

 Crab emerges. As 

 the Sponge grows 

 still further in thick- 

 ness the margin over- 

 lapping the aperture 

 of the shell expands, 

 leaving a conical 

 cavity leading from 

 the exterior to the 

 shell, surrounded, of 



FIG. 38. Section through a sponge (D) rollr p V, v ftnon^p 

 showing A, the little shell; B, the COL IS6 > DV . ^P on g e 

 worm ; C y the Hermit-crabs in their structure, in which 

 natural positions. , i /-* -\ i mi 



the Crab lives. Thus 



as the Hermit-crab increases in size it is ever pro- 

 vided with a wider hole to accommodate its body 

 by the growth of the Sponge, and the little shell 

 wholly deserted remains as a token of the past 

 history of the pair. But in the later stages of 

 growth a third creature is taken into the partner- 

 ship, in the person of a small segmented Worm 

 which lives in the hole with the Hermit-Crab. 

 The need for this third person seems to be one 

 of a sanitary character. The cleanliness of the 



