THE ANCESTOR OF THE TRILOBITE. I4 1 



state of preservation, and was in that respect intermediate between the trilobite and the 

 theoretical ancestor. Every analysis of the cephalon of the trilobite shows that it is made 

 up of several segments, certainly five, probably six, possibly seven. Every study of the tril- 

 obite, whether of adult, young, or protaspis, indicates the primitiveness of the lateral exten- 

 sions or pleural lobes. The same studies indicate as clearly the location of the vital organs 

 along the median lobe. These suggestions all point to a soft-bodied, depressed animal composed 

 of few segments, probably with simple marginal eyes, a mouth beneath the anterior margin, 

 tactile organs at one or both ends, with an oval shape, and a straight narrow gut running 

 from anterior mouth to terminal anus. The broad flat shape gives great buoyancy and is 

 frequently developed in the plankton. Inherited by the trilobites, it proved of great use to 

 the swimmers among them. 



The known animal which most nearly approaches the form which I should expect the 

 remote ancestor of the trilobites to have had is Amiskwia sagittiformis Walcott (Smith- 

 son. Misc. Coll., vol. 57, 1911, p. 112, pi. 22, figs. 3, 4). This "worm" from the Middle 

 Cambrian is similar in outline to the recent Spadella, and is referred by Walcott to the 

 Chjetognatha. It has a pair of lateral expansions and a flattened caudal fin, a narrow 

 median alimentary canal, and a pair of rather long simple tentacles. With the exception 

 of a thin septum back of the head, no traces of segmentation are shown. 



Some time in the late pre-Cambrian, the pre-trilobite, which probably swam by rhyth- 

 mic undulations of the body, began to come into occasional contact with a substratum, and 

 two things happened : symmetrically placed, i. e., paired, appendages began to develop on the 

 contact surface, and a test on the dorsal side. The first use of the appendages may have 

 been in pushing food forward to the mouth, and for the greater convenience in catching 

 such material, a fold in front of the mouth may have elongated to form the prototype of 

 the hypostoma. At this time the substratum may not have been the ocean bottom at all, 

 but the animals, still free swimmers, may have alighted at feeding time on floating algae 

 from the surface of which they collected their food. While the dorsal test was originally 

 jointed at every segment, the undulatory mode of swimming seems to have given way to the 

 method of sculling by means of the posterior end only, or by the use of the appendages, and 

 the anterior segments early became fused together. 



The result of the hardening of the dorsal test was of course to reduce to that extent 

 the area available for respiration, and this function was now transferred in part to the limbs, 

 which bifurcated, one branch continuing the food-gathering process and the other becom- 

 ing a gill. The next step may have been the "discovery" of the ocean bottom and the 

 tapping of an hitherto unexploited supply of food. Upon this, there set in those adapta- 

 tions to a crawling mode of existence which are so well shown in the trilobite. The crawl- 

 ing legs became lengthened and took on a hardened test, the hypostoma was greatly elongated, 

 pushing the mouth backward, and new segments were added to produce a long worm-like 

 form which could adapt itself to the inequalities of the bottom. That the test of the appen- 

 dages became hardened later than that of the body is shown by the specimens of Neolcnus, 

 in which the dorsal shell as preserved in the shale is thick and solid, while the test of the 

 appendages is a mere film. 



The late Proterozoic or very earliest Cambrian was probably the time of the great 

 splitting up into groups. The first development seems to have been among the trilobites 

 themselves, the Hypoparia giving rise to two groups with compound eyes, first the Opis- 

 .thoparia and later the Proparia. About this same time the Copepoda may have split off 



