86 HISTORICAL PALEONTOLOGY. 



able stalks (as they are in the existing lobsters and crabs) ; and 

 in some of the Cambrian Trilobites, such as the little Agnosti 

 (fig- 3i> 9}, the animal was blind. The lateral portion of the 

 head-shield are usually separated from the central portion by 

 a peculiar line of division (the so-called " facial suture ") on 

 each side; but this is also wanting in some of the Cambrian 

 species. The backward angles of the head-shield, also, are 

 often prolonged into spines, which sometimes reach a great 

 length. Following the head-shield behind, we have a portion 

 of the body which is composed of movable segments or " body- 

 rings, " and which is technically called the " thorax. " Ordi- 

 narily, this region is strongly trilobed, and each ring consists of 

 a central convex portion, and of two flatter side-lobes. The 

 number of body-rings in the thorax is very variable (from two 

 to twenty-six), but is fixed for the adult forms of each group of 

 the Trilobites. The young forms have much fewer rings than 

 the r ull-grown ones; and it is curious to find that the Cam- 

 brian Trilobites very commonly have either a great many rings 

 (as in Paradoxides, fig. 31, a), or else very few (as in Agnostus, 

 fig- 3i g}- In some instances, the body-rings do not seem to 

 have been so constructed as to allow of much movement, but 

 in other cases this region of the body is so flexible that the 

 animal possessed the power of rolling itself up completely, like 

 a hedgehog; and many individuals have been permanently 

 preserved as fossils in this defensive condition. Finally, the 

 body of the Trilobite was completed by a tail-shield (technic- 

 ally termed the "pygidium"), which varies much in size and 

 form, and is composed of a greater or less number of rings, 

 similar to those which form the thorax, but immovably amalga- 

 mated with one another (fig. 31, h). 



The under surface of the body in the Trilobites appears to 

 have been more or less entirely destitute of hard structures, 

 with the exception of a well-developed upper lip, in the form 

 of a plate attached to the inferior side of the head-shield in 

 front. There is no reason to doubt that the animal possessed 

 legs; but these structures seem to have resembled those of 

 many living Crustaceans in being quite soft and membranous. 

 This, at any rate, seems to have been generally the case; 

 though structures which have been regarded as legs have been 

 detected on the under surface of one of the larger species of 

 Trilobites. There is also, at present, no direct evidence that 

 the Trilobites possessed two pairs of jointed feelers (" an- 

 tennae") which are so characteristic of recent Crustaceans. 



