FREE MESSMATES. 45 



nematode, to which he has given the name of Odontobius, 

 and which lives on the palatal membranes (the whale- 

 bones) of the southern whale. It is evidently a mess- 

 mate. It can get nothing from the whalebones, but it 

 snaps up on their passage in the interstices of the 

 baleen, small animals of all kinds which swarm in these 

 waters. When we open the Pylidium girans, we often 

 find in the interior of its digestive cavity a larva, which 

 was once thought to be descended from it, but instead 

 of being allied to the Pylidium, this larva comes from a 

 nemertian known by the name of Alardus caudatus. 

 The young nemertian never abandons his host until it 

 approaches the period of puberty, and then all the in- 

 dividuals living under the same conditions emancipate 

 themselves at once, to pass the rest of their days free 

 and roving like their mother. 



Worms which have less freedom, like the Distomians, 

 are sometimes both messmates and parasites. We 

 find a remarkable example of this in the Distomwn 

 ocreatum of the Baltic. According to the observations of 

 Willemoes-Suhm, this trematode passes its cercarial life 

 freely in the sea, and instead of encysting itself in the 

 body of a neighbour, it attaches itself to a copepod 

 crustacean, the whole of the inside of which it devours, in 

 order to clothe itself afterwards with the carapace of its 

 victim. It is under the cover of its prey that it passes 

 into the herring, and completes its sexual evolution. 



Mons. Ulianin has recently found another Distome 

 (Distomum ventricosum) which passes its cercarial life in 

 freedom in the bay of Sebastopol, and completes its evolu- 

 tion in the fishes of the Black Sea. J. Muller has long 

 since found Cercaria living freely in the Mediterranean. 



