334 PALAEOSTRACA. 



appendage which may be regarded as an exopodite, on the sixth 

 pair of hmbs (Fig. 158, .r, p. 345), as well as in the form of the 

 abdominal limbs (a^, «.,)• 



While the Palaeostraca in one direction join on to the Crustacea 

 and to their liypothetical ancestors, the Protostraca, they are further 

 of great interest as probably the original group from which the 

 air-breathing Arachnida developed. The view that the Arachnida, 

 and above all the Scorpiones, are closely related to Limulus — first 

 put forth by Strauss-Durkheim, and more recently established on 

 a firmer basis by Kay Lankester (No. 16) — appears supported by 

 so many points of agreement in the structure and the development 

 of the two groups, that we cannot refuse to accept it; it will be 

 more fully discussed below. 



Only a few of the ontogenetic stages of the fossil Palaeostraca 

 have been preserved through the favourable character of the stone 

 enclosing them. The ontogenetic stages of many Trilobites are, 

 however, known, and in some cases it has been possible to put 

 together complete series, so that we are able to solve many important 

 problems in the metamorphosis of the Trilobites. The ontogenetic 

 stages of tlie Gigantostraca, on the contrary, have not as yet been 

 observed. 



I. Trilobita. 



Notwithstanding the more recent researches of Ford (Nos. 2 and 

 3), Walcott (No. 6), Matthew (No. 4), the earlier investigations of 

 Barrande (No. 1), which are here followed, still provide the founda- 

 tion for our knowledge of the metamorphosis of the Trilobites.* 

 Barraxde distinguished four ontogenetic types which, however, he 

 considered as merely provisional. Three of these methods of 

 development may be regarded as modifications of one type, while 

 the Agnostus type seems quite distinct from the others. 



1. Type in which the adult pygidium develops late. 

 A very complete metamorphosis, consisting of many consecutive 

 stages, was established by Barrande for Sao liirsuta (Fig. 150), a 

 form belonging to his first ontogenetic type. The youngest known 

 stages (^4) still difl"er greatly in apj^earance from the adult. They 

 are exceedingly small (two-thirds of a millimetre in diameter), 

 rounded, and disc-shaped, and as yet show no distinct segmentation 

 of the body. The latter consists chiefly of the rudiment of the 

 future cephalic region, in which the glabella is already marked 

 ofi' from the cheeks by dorsal furrows. The posterior border of the 

 * [See Beecher, Append. Lit. Trilobita, No. I. — Ed.] 



