88 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Gurney (Quart. Jour. Mic. Sci., vol. 46, 1902, p. 462) supports Weldon in the conclusion 

 that the long spines of the zosea are directive, and states that the animal swims in the direction 

 of the long axis of the spine. He also suggests that, since the period of their presence cor- 

 responds to the period before the development of the "auditory" organs, the spines may 

 perform the functions of balancing and orientation. It is generally admitted that the spine 

 of the zosea is also protective, and the obvious function, first pointed out by Spence Bate 

 in 1859, is that it contains a ligament which helps suspend the heart, which lies beneath the 

 spine. This latter function may have been that of the median tubercle in the trilobite. Such 

 an explanation would account for the backward migration mentioned above, for as the 

 stomach enlarged and the mouth moved backward on the ventral side, the heart may have 

 been pushed backward on the upper side. 



There is also a curious parallelism between the ontogenetic history of the zoieal spine 

 and the phylogenetic history of the Trinucleidae or Cheiruridae (Nieszkowskia is the ancient 

 member of this family in which the spine replaces the tubercle). When first hatched, the 

 larval crab shows no trace of the spine, but very quickly it evaginates, lying dorsally on the 

 median line, pointing forward (Faxon, Bull. Mus. Comp. Zool., vol. 6, 1880, pi. 2). With 

 the splitting of the original envelope, the spine becomes erect, but persists only a short time, 

 and is reduced to a vestigial tubercle toward the end of the zoosal stages, its disappearance be- 

 ing, as pointed out by Gurney, coincident with the development of the balancing organs. This 

 manner of suspension of the heart by a long tendon certainly does suggest that Gurney is 

 right in his interpretation of the function. Briefly, the zoaeal spine served for a short time 

 a function later taken over by other organs. It was not present in the youngest stages, it 

 became prominent at a very early stage, was soon vestigial, and then lost. 



Take now the trilobites. There is no trace of the median pustule in the protaspis of 

 any form, and in many primitive trilobites it is absent. It appears first as a long spine in 

 certain families, and later becomes vestigial and disappears. Very few trilobites of Silurian 

 and later times show it at all. 



In the particular case of the Trinucleidse, which were burrowers, the spine is present 

 on only the oldest and most primitive of the group, a form which has only a most rudimen- 

 tary fringe. It is obvious from the large size of the pygidium in the larval trinucleid that 

 this family is derived from a group of free swimmers. Trinucleoides reussi was perhaps in 

 the transitional stage, just leaving the swimming mode of life, and belonged to a group 

 which had not developed any other "statocyst" than the median spine. Among the later 

 Trinucleidaa the spine became a vestigial tubercle, and in some cases entirely disappeared. A 

 similar history can be traced in the Cheiruridae, starting from some such forms as the Ameri- 

 can Lower Ordovician Nieszkowskia (N- perforator p. ex.). 



Another example of a median spine instead of a tubercle is in Goldius rhinoceros (Bar- 

 rande). Since this species is not from the oldest Goldius-bearing rocks, but from the Lower 

 Devonian, it does not follow what seems to be the general rule, but makes an interesting ex- 

 ception. Goldius rhinoceros (Barrande) (Supplement, 1872, pi. 9, figs. 12, 13) has the 

 median tubercle elevated into a stubby, recurved spine very suggestive of the horn of a 

 rhinoceros. Since the eyes of this species are very well developed, there seems no especial 

 reason for the elevation of a parietal eye, and the example certainly does not support that 

 interpretation. 



3. This tubercle is essentially similar to other tubercles on the median line of cephalon, 

 thorax, and even pygidium. This has been discussed sufficiently under section i above, but 



