6/2 THE POPULAR SCIENCE MONTHLY. 



labor, jjrefer to install themselves in the mouth of a neighbor, and take 

 tithes of the morsels which he swallows. 



The little crab that makes its abode within the shell of the edible 

 oyster (Fig. 2) is a true messmate, and the oyster is but one of many 

 bivalve mollusks that give shelter and partial support to these di- 

 minutive crustaceans. These crabs, called by naturalists Pinnotheres, 

 though in one sense dej^endents, are at the same time of great service 

 to the animals within whose shells they receive protection. Van 

 Beneden says of them: "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 



Fig. 2. Oyster Ceab.> 



the advantage of both ; the remains of food which the pinnothere 

 abandons are seized upon by the moUusk. It is the rich man who in- 

 stalls himself in the dwelling of the poor, and enables him to participate 

 in all the advantages of his position. The pinnotheres are, in our opin- 

 ion, true messmates. They take their food in the same waters as their 

 fellow-lodgers, and the crumbs of the rapacious crabs are doubtless 

 not lost in the mouth of the peaceful mussel. . . . 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, they choose admirably the moment to 

 rush out to the attack, and always fall on their enemy unawares. 

 Some pinnotheres live in all seas, and inhabit a great number of bi- 

 valve mollusks." 



In the examples thus far cited, and in many more that have been 

 observed, the dependent forms are free to depart whenever they 

 choose, and are therefore called free messmates. Though for a time 

 giving up their liberty, they sooner or later resume it, in possession of 

 all their organs for fishing and locomotion, and in all respects fitted 

 to live an independent life. There are others, however, that enter 

 into the same sort of association, and make the relation a permanent 

 one : these are known as fixed messmates. They are free in their 

 youth, but, as maturity approaches, and the cares of a family are 

 thought of, a host is selected in which they establish themselves, and, 

 throwing aside their fishing and locomotive apparatus, they renounce 

 the world, and even part with the most precious organs of animal life, 

 not excepting those of the senses. 



' From Morse's " First Book of Zoology." 



