180 Observations on Triiobites. 



him to endeavour to find something parallel among the Crus- 

 tacea. He has traced the rudiments of this division in the 

 Palinurus, Gammarus, Talitrus, Cymothoa, Ligia, and if 

 might doubtless be traced in many others. 



The objection to their being Crustacea, derived from the 

 fact that they have neither antenna? nor locomotive organs, is 

 more serious. If they had ever been provided with these parts 

 they would surely have been discovered ere this : but neither 

 the researches of naturalists in Europe, nor our own ob- 

 servations, have exhibited the slightest indication of their 

 presence ; but in the order Gymnobranchia, the antennae are 

 either very small or disappear ; and the feet changed into 

 natatory organs or gills, lose their solidity. If the Triiobites 

 then are supposed to have belonged to this order, we may 

 look in vain for feet or true antennae. 



It is probable that the lateral lobes are the true organs of 

 locomotion ; and this opinion receives considerable support 

 from the following considerations. We shall afterwards en- 

 deavour to show that they also may hold the branchial appa- 

 ratus. 



In those specimens of the Isotelus in the Cabinet, which are 

 contorted or doubled up upon themselves, the extremities of 

 the lateral lobes are found to be diminished in size, as if they 

 were folded under each other, while the parts near the middle 

 lobe retain more or less their natural position. This would 

 seem to indicate that the extremities were free, and of course 

 served as organs of locomotion. They may also have served 

 to seized their prey ; in this they were possibly assisted by the 

 tail. In other specimens, however, we have undoubted evi- 

 dence of this structure. In these, which appear as if a strong 

 power had been impinged upon the middle lobe, the lateral 

 ones are forced so widely apart that they can be examined and 

 even measured. A reference to the figure of the Mpheus 

 aculeatus and others, as given in the appendix to Parry's 



