SUB-CLASS I TRILOBITA 617 
the joints become flattened and leaf-like, carrying tufts of setae, and being 
adapted for swimming. 
The exopodites are fringed along their posterior edges with narrow, 
oblique lamellar elements becoming filiform at the ends, thus converting the 
limb into a swimming organ, and also serving respiratory functions (Figs. 
1267, 1270). 
Halits.—In the absence of any closely allied recent forms, it is difficult to 
reach definite conclusions respecting the manner of life of Trilobites, except 
such as are based upon their organisation and mode of occurrence. They were 
undoubted marine animals, since their remains are found only in salt-water 
deposits, associated with Brachiopods, Cephalopods, Crinoids, and other typical 
oceanic forms. Some species are plentiful in calcareous or argillo-caleareous 
deposits, with thick-shelled Brachiopods, Gastropods, and reef-building Corals, 
which evidently did not live at any considerable depth. Other forms appear 
to have been bottom crawlers, frequenting either muddy or sandy bottoms ; 
and again, others like 7’rinucleus, lived partly buried in the soft mud. On the 
other hand, many species indicate, from the absence of visual organs, a com- 
paratively deep-water habitat. The structure of the appendages of many was 
probably such as to permit of both swimming and crawling, as in a number of 
families of modern Crustacea, and they were therefore restricted neither to the 
shore nor to the bottom. This doubtless explains the occurrence of the same 
species in very different sediments. 
Power of Enrolment.—The bodies of most Trilobites were capable of being 
rolled up completely like many of the Isopods (Fig. 1271). In the enrolled 
condition the margin of the pygidium is closely applied 
to the doublure of the cephalon, thus entirely concealing 
the ventral side of the body. The thoracic segments 
overlap, and admit of more or less motion upon one 
another. The pleura also imbricate, and their fuléra 
are provided with facets upon which the fulera of 
adjacent segments impinge. The ends of the pleura 
thus protect the ventral surface along the sides, when 
the animal is enrolled. Some forms appear to have 
possessed the power only to a limited degree. In Ge ee eore: 
these, the creature is usually found extended, and the Enrolled specimen. 
facets on the fulcra are either rudimentary or absent. 
Ontogeny.—Minute spherical or ovoid fossils associated with Trilobites 
have been described as possible Trilobite eggs, but nothing is known, of 
course, of the embryonic stages of the animals themselves. The smallest and 
most primitive organisms which have been detected, and traced by means of a 
series of specimens through successive changes into adult Trilobites, are little 
discoid or ovate bodies not more than 1 mm. in length. This first larval form 
has been named the protaspis (Beecher), and has been found to be the normal 
larval type characteristic of all Trilobites. It is believed to approximate the 
protonauplius form, or the theoretical, primitive, ancestral, larval form of the 
Crustacea. 
The simple characters possessed by the protaspis are the following, as 
drawn from the study of this stage in all the principal groups of Trilobites :— 
Dorsal shield minute, not more than 0-4 to 1 mm. in length ; circular or ovate in 
form ; axis distinct, more or less strongly annulated, limited by longitudinal grooves ; 

