MOLLUSCS AS MESSMATES 173 
As with Fishes (see p. 171), Bivalve Molluscs are not always 
lodgers in the case of partnerships, but may afford shelter to 
weaker creatures. A well-known instance is that of the little 
rounded Lodger-Crabs (Pzxnotheride), in which the eyes have 
undergone great reduction. Among bivalves which provide them 
with homes may be mentioned Horse-Mussels (JJodzo/a), Oysters 
(Ostrea), Pinnas, and Tridacnas, while some crabs of the kind 
take up their quarters within ascidians or sea-cucumbers. Van 
Beneden thus speaks of these little lodgers (in Animal Parasites 
and Messmates):—‘It is not a taste for voyaging which tempts 
them, but the desire of having always a secure retreat in every 
place. The pinnothere is a brigand who causes himself to be 
followed by the cavern which he inhabits, and which opens only 
at a well-known watchword. The association redounds to the 
advantage of both; the remains of food which the pinnothere 
abandons are seized upon by the mollusc [or, rather, some of its 
remains may be carried by ciliary action into the mouth of the. 
mollusc]. It is the rich man who instals himself in the dwelling 
of the poor, and causes him to participate in all the advantages 
of the position. The pinnotheres are, in our opinion, true mess- 
mates. They take their food in the same waters as their fellow- 
lodger, and the crumbs of the rapacious crabs are doubtless 
not lost in the mouth of the peaceful mussel. There is no 
doubt that these little plunderers are good lodgers, and if the 
mussels furnish them with an excellent hiding-place and a safe 
lodging, they themselves profit largely by the leavings of the 
feast which fall from their pincers. Little as they are, these 
crabs are well furnished with tackle, and advantageously placed 
to carry on their fishery in every season. Concealed in the 
bottom of their living dwelling-place (a den which the mussel 
transports at will) they choose admirably the moment to rush 
out to the attack, and always fall on their enemy unawares. 
Some of these pinnotheres live in all seas, and inhabit a great 
number of bivalve molluscs.” The habits of these curious little 
crabs attracted attention in remote times, and have been the 
subject of much curious speculation. Stebbing (in 4 History of 
Crustacea) makes the following remarks upon the ancient views, 
and discusses the origin of the commensal habit:—“ The name 
Pinnoteres means one that watches or guards the Pzxna, and 
there can be little doubt that it was the form used by Aristotle 
