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 (Pinnot her idee], in which the eyes have 

 undergone great reduction. Among bivalves which provide them 

 with homes may be mentioned Horse- Mussels (Modiold), 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 A 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 Pinna, and 

 there can be little doubt that it was the form used by Aristotle 



