48 AFFINITY OF THE TRILOBITES 



latter be the case, the young never receive a protecting covering ; if, however, the parents 

 have shells, then the first traces of it are seen immediately after the first moult, in the form 

 of a membranous fold, which issues from the neck, and which at once covers one half of the 

 body. The young animal, after this period, has become considerably larger, and its body has 

 become thicker and exhibits lateral notches behind the third organ of locomotion, from wliich 

 the feet gradually develop themselves. The little animal still retains its first three organs 

 of locomotion in an unchanged form as long as the feet remain imperfect ; as soon, however, 

 as the feet (which grow laterally from the body) have attained their relative development, 

 in proportion to the dimensions of the animal, the former organs of locomotion begin to 

 diminish, and are gradually metamorphosed into the form exhibited in the full grown 

 animal, which is frequently very rudimentary. It can then be distinctly seen that they are 

 the true tentacula of the developed animal, and that according to their size in the subsequent 

 stages of existence .they are more or less diminished or modified. The formation of the eye 

 progresses equally with this metamorphosis. At first it is a simple small point or dot, and 

 first appears about the time when the anterior half of the thoracic feet have formed them- 

 selves into isolated organs ; but, besides this little dot, there are a couple of other dark spots 

 in the head, which simultaneously and gradually with the other organs form themselves into 

 compound eyes. A very short period of time is required for the formal development of the 

 rest of the animal, for the subsequent structures are already conveniently isolated and per- 

 fectly formed when the young animal has scarcely reached the hundredth part of the 

 dimensions it is afterwards to attain. From this time forward, therefore, it advances regularly 

 in its growth, and casts its membranes from time to time, at longer or shorter intervals, as 

 the rapidity of its growth may demand. The cast-off membranes are split along the whole 

 length of the back, and the animal draws forth all parts from this opening gradually in 

 such a manner that even the smallest hair remains affixed to the old membrane and forms 

 itself anew on the new membrane. The cast-off membrane, however, consists only of the 

 epithelium, never of the real cutis, and it is therefore ver}^ thin and delicate. The Trilobite 

 remains which are said to have been produced from such exuviae are thus probably mere 

 impressions, and do not exhibit remains of the shell itself, since distinct evidence of its 

 having been cast off would have been preserved in a longitudinal section along the centre 

 of the back, and such a section not having been yet observed in any Trilobite, I consider the 

 opinion expressed by Wahlenberg, that in many supposed Trilobite shells we have only the 

 cast-off membranes before us, to be incorrect. 



It may still be thought probable, however, from the nature of the metamorphosis of the 

 Phyllojmda, that Trilobites experienced a similar change, nor do I at all question the 

 possibility of such a view being correct. On the contrary, I am inclined to recognize the 

 vestiges of such early stages of the animal's growth in the obscure shields which have been 

 supposed the types of new genera, under the names Agnodus and Batfus ; for, on carefully 

 examining specimens of the former, we find not only a singular variation in size, occurring 

 as they do from the dimensions of a mustard seed to that of a pea, but also a perceptible 

 difference in the form of their constituent parts, and I think I can distinguish four different 

 forms, which may be brought under two groups. Before, however, explaining these, I shall 

 describe the Ai/nostus shield in general, referring for illustration to the representations. 



