146 PACKARD—CLASSIFICATION OF ARTHROPODA, [April 3, 
the Crustacea.’ I also add what appear to be the essential characters 
of the phylum. 
Phylum I, Pat&opopa. Composed of three classes—/. ¢., 
Trilobita, Merostomata, Arachnida. 
Body trilobate (in Trilobita and many Merostomata), never pro- 
tected by a true carapace, composed of a head and trunk region; 
the head-region separate from the trunk, in Trilobites (Triarthrus) 
composed of (judging by the appendages) five segments (som- 
ites, arthromeres), in Merostomes six, while in Arachnida the head 
fused with the so-called thorax (cephalo-thorax) also consists of six 
segments. The first pair of head-appendages, in a single trilobitic 
genus (Triarthrus), are long, slender, uniaxial and antenniform, or 
biramose, chelate (Merostomata and Arachnida) ; all the post-oral 
appendages, in the most primitive class (Trilobita), biramose, con- 
sisting of an outer and inner many-jointed division, but all homon- 
omous, or retaining the same fundamental and primitive shape from 
the mouth to the end of the body, and never (as they are in Crus- 
tacea) differentiated into true or functional mandibles, maxille, 
maxillipedes, ambulatory uniaxial thoracic legs, or biramose 
abdominal limbs. The gnathobases, or coxal joint of each limb, 
especially those near the mouth, armed with inward projecting 
spines, acting as jaws to tear and to keep the food or prey from 
escaping. In the Merostomata the post-cephalic or trunk (abdomi- 
nal) limbs biramose and adapted for swimming, and either (in 
Trilobites) expanded posteriorly and probably serving both for 
swimming and respiration, or in Merostomes (Zzmu/us) bearing on 
the exopodite of each limb, except those of the first pair, a pile of 
numerous gill-sacs. In Arachnida, in adaptation to a terrestrial life, 
the six pairs of abdominal or trunk limbs are reduced, mostly 
atrophied, represented in the scorpion by the pectines and the four 
pairs of invaginated book-lungs, and in spiders by the two pairs of 
book-lungs (Mygale) and the three pairs of spinnerets, which are 
2-3 jointed, external free appendages. A hypostoma is present 
and well developed in Trilobita and Merostomata, as also a double 
underlip, the chilaria of Limulus. 
The eyes of -Asaphus, etc., and of Limulus are compound, al- 
1 Paleeocarida was proposed when I believed that Limulus and its allies were 
Crustacea; my name Podostomata was proposed for a greup embracing the two 
classes Trilobita and Merostomes; the present name, Palzopoda, is needed to 
embrace the three classes mentioned. 
