NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I37 



fertilization of the eggs was probably external. Postembryonic de- 

 velopment was anamorphic. The first major diversification of the 

 Protarthropoda gave rise to the ancestors of the Trilobita and the 

 ancestors of the Mandibulata (fig. 54). 



10. — The Trilobita preserved the uniform, generalized structure 

 and segmentation of the protarthropod appendages, but otherwise they 

 became highly specialized by a lateral extension of the margins of 

 the body segments, taking on thus a broad, flattened form except for 

 a median elevation giving passage to the alimentary canal. Further- 

 more, the first four postoral segments became intimately united with 

 one another and with the prostomial acron to form a solid anterior 

 body section, or prosoma, the so-called "head," bearing the labrum, 

 the eyes, the antennules, and four pairs of postoral ambulatory 

 appendages. Basal endites of the anterior appendages may have 

 served as feeding adjuncts, but the trilobites, so far as known, devel- 

 oped no specific jaws. The Trilobita were entirely marine animals, 

 but they lived at the bottom of the water, and their legs show few 

 deviations from the ambulatory type of structure, except for the high 

 development of branchial lobes from the lateral surfaces of the 

 coxopodites. The extended tergal margins covering the gills probably 

 formed respiratory chambers. The position of the genital openings 

 in the trilobites has not been discovered, but, because of the close 

 relation between the Trilobita and the Chelicerata, the genital aper- 

 tures may be expected to be found on the fourth postcephalic segment. 

 The Trilobita became extinct by the end of the Paleozoic period of 

 geological history, but from a branch of the primitive pre-Cambrian 

 prototrilobites were evolved the Chelicerata. 



II. — The Chelicerata are distinguished from the Trilobita by the 

 union of several additional somites with the head to form a more 

 extensive prosoma, by the loss of the acronal appendages (anten- 

 nules), by a greater differentiation among the somatic appendages, 

 and by the forcipate structure of the reduced first appendages. Very 

 commonly, also, there is an extra podomere in at least some of the 

 legs, the patella, interpolated between the femur and the tibia. In 

 modern forms the nephridial organs are suppressed in most of the 

 somites, but some of them are retained as coxal glands, and (except 

 in Pycnogonida and some Acarinida) the genital openings occur 

 always on the eighth postoral somite. The Chelicerata have become 

 the most sepecialized of all the arthropods, there being little in their 

 body form and general organization suggestive of the ancestral centi- 

 pede type of structure, which is so evident throughout the mandibulate 



