io6 THE BIOLOGY OF THE SEA-SHORE 



from the mouth or through tiny pores (" cinclides ") in the 

 wall of the column. 



Structures of a similar nature to the nematocysts of 

 Coelentera occur in several Turbellarian and Nemertine 

 worms, but it is doubtful whether they are homologous with 

 the Ccelenterate forms. The undoubted occurrence of 

 nematocysts in the cerata of Nudibranch (^^olidae) molluscs 

 has given rise to a great deal of controversy, but the researches 

 of Strethill Wright, Grosvenor, and Cuenot have defini- 

 tively proved that these are derived from the anemones on 

 which the molluscs feed. The nematocysts pass through 

 the stomach and hepatic diverticula without being digested. 

 Eliot {op. cit.) summarises the evidence which has led to 

 this conclusion as follows : — 



1. Several kinds of nematocysts may occur in one i^olid, 

 and individuals of the same species may have different kinds 

 of nematocysts. 



2. It has been shown experimentally that the cnidosacs 

 always contain nematocysts similar to those of the Coelentera 

 on which the .^olid has been feeding, and that the 

 nematocysts found in the hepatic diverticula and faeces are 

 similar. 



3. The nematocysts of an ^olid can be changed by 

 changing its food, and those -Solids which feed on animals 

 devoid of nematocysts have none themselves. 



The use of the nematocysts by the i^olid seems to be 

 purely fortuitous ; there is no evidence that they are dis- 

 charged against advancing enemies. On being attacked the 

 animal contracts its body, at the same time erecting and 

 sometimes lengthening its cerata so that the assailant can 

 hardly help touching them. When the cerata are squeezed 

 or bitten the stinging nematocysts are liberated ; the 

 assailant probably withdraws and the ^olid escapes with 

 the loss of a few cerata, a fact which seems to cause it little 

 inconvenience. Apparently those i^olids which have no 

 nematocysts, e.g. Calma and Fiona, have specially abundant 

 mucus or offensive glands. 



In connection with this partly mechanical, partly 



