CLASSIFICATION OF CRUSTACEA. 15 



and have their closest analogy among Crustacea with the Choristopods, 

 especially the Caprellidae, the joints of the body being distinct, and 

 the legs long and ambulatory. 



Trilobites. — With regard to the position of Trilobites in an arrange- 

 ment of Crustacea, we offer the following observations. 



In Apus and Limnadia we have examples of species with an abnor- 

 mal number of segments, and foliaceous organs of locomotion concealed 

 below. The absence of pediform jointed appendages among all exam- 

 ples of fossil Trilobites is proof that there were no such appendages 

 when living, as they could not have escaped preservation. A shell of a 

 texture durable enough to be preserved, must have existed on legs suffi- 

 ciently large to correspond in size with many Trilobites ; for the articu- 

 lation in all Crustacea legs is made by processes in the shelly covering 

 of the legs, these being the only firm parts ; and such articulations for 

 large legs would require a firm exterior, or else the member would be 

 little better than a flabby piece of flesh, even if it had articulations. It 

 is clear, therefore, that the organs of locomotion must have been folia- 

 ceous, as in Limnadia. Such organs would be sufficient for swimming, 

 and would not interfere with the adhesion of the animal, Chiton-like, 

 to any body at hand. The antennas in some species may have been 

 elongated, jointed organs, as specimens have shown. 



Considering these points as established, does it follow that the 

 species were properly Entomostracans related to the Phyllopoda ? 

 One great distinction separating the Entomostraca and Choristopods 

 — between which groups the Trilobita are supposed by authors to lie — 

 consists in the existence of a regular series of organs below the abdo- 

 men in the latter, and the absence of such organs in the former. Even 

 in Limnadia and Branchipus, the abdomen has no such series of 

 organs.* This part of the body in the Pbyllopods is often very short, 

 as in Limnadia, or narrow, as in Branchipus, while in many Tri- 

 lobites, as the genus Isotelus, it is very broad and large, — so large, in 

 fact, that we can hardly refuse to believe that it was provided with 

 leaflets below, either like those of the thorax, or more properly bran- 

 chial in character. In many species there is no obvious line between 

 the thoracic and abdominal joints, as is true of some Isopoda, while in 



* The large posterior segment of the Limulus, with which the caudal appendage is 

 articulated, and which covers foliaceous appendages, appears to be cephalotboracic and not 

 abdominal, and the so-called tail in the common species is, therefore, all that exists of 

 the abdomen. 



