1903.] PACKARD—-CLASSIFICATION OF ARTHROPODA. 147 
ways sessile, and distinguished by the thick, either lenticular or 
long conical lenses, arranged in quincunx order. 
_ The integument.is chitinous, insoluble as in insects, never con- 
taining carbonate or phosphate of lime, or forming a solid crust 
as in the higher Crustacea. The cartilaginous plate (endoster- 
nite), so large and well developed in Limulus, is also present in 
Arachnida. 
In the living forms (Limulus and Arachnida) the digestive canal 
may be differentiated into a slender cesophagus, a proventriculus 
armed with rows of numerous chitinous teeth (Limulus) and an 
intestine, the stomach being but slightly differentiated. The liver 
or hepato-pancreas is large and voluminous. In the Merostomes 
(Limulus) there are no salivary glands, though occurring in Arach- 
nida. Genital openings always (Merostomata and Arachnida) proso- 
goneate, the oviducts or seminal ducts opening out separately on the 
posterior aspect of the basal abdominal limbs (Limulus), or in 
Arachnida united into a single terminal passage, opening bya single 
orifice at the base of the abdomen. In the marine forms, with gills 
or localized respiration, the heart is tubular and the arterial system 
remarkably developed and finely divided, whereas in the tracheate, 
terrestrial forms the arteries and veins are absent, respiration being 
carried on throughout the body (chiefly abdominal) cavity. 
In the Palzeopoda there is no true metamorphosis like chat of the 
Crustacea, no nauplius or zoéa stage. The first or earliest larval stage, 
the profaspis’ stage of Beecher, can, so far as we can see, in no way 
be likened to the nauplius of a crustacean. The nauplius has an oval 
body, not differentiated into segments, but with three pairs of 
slender swimming limbs, which finally become the two pairs of 
antenne and the mandibles of the adult. In the protaspis of trilo- 
bites, as defined by Beecher, the conditions are entirely different 
and such as suggest the origin from a polymerous Annelid ancestor. 
The minute disk-like or suborbicular larva of different genera of 
Trilobites described by Barrande and by Beecher consist of two 
regions, a head and trunk or abdomen. There are in the head 
indications of five annulations, the same number as in the adult 
Triarthrus; the much shorter abdominal region has from ‘‘ one 
1 This term was proposed by Beecher in his paper on ‘‘ The Larval Stages of 
Trilobites” (Amer. Geologist, September, 1895). Previously to that, A. C, 
Oudemans, in 1886, in the article cited, proposed the name /roagnostus for the 
same stage. If used, this name might be amended to read Protagnostus stage. 
