156 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
In 1885 and 1886 (Annals and Mag. Nat. Hist., July, 1886) Claus regarded the Gigantostraca 
as a class intermediate between the Crustacea and Arachnida. He thinks that the Arachnida 
descended from the Gigantostraca, adding, “I by no means affirm the Arachnoidal nature of 
Limulus.” 
In 1879, in our Text-book of Zoology, as the result of Milne Edwards’s researches, we divided 
the Crustacea into two subclasses, the Neocarida and Paleocarida, the latter group comprising the 
Merostomata and Trilobita. In a previous paper we had shown the close homologies of the eye of 
Trilobites to the compound eyes of Limulus. 
In April, 1881, Mr. C. D.Waleott (Bull. Mus. Comp. Zool., viii, No. 90, p. 209), under the class 
Peecilopoda, places two subelasses, viz, Merostomata and Palade (Trilobita), giving definitions 
of the groups. 
In 1881, in his article “Limulus an Arachnid” (Quart. Journ. Mier. Se.) Prof. E. Ray Lan- 
kester proposed the term Hematobranchia, which he regarded as the equivalent of Merostomata. 
This group of the class Arachnida, as understood by Lankester, embraces the three orders: 1, 
Trilobita; 2, Eurypterida; and 3, Xiphosura. 
In 1885 (Embryology of Limulus Polyphemus, I1I, Proc. Am. Phil. Soc., January, 1885), we 
referred Limulus, with the Eurypterida and Trilobita, to a class by themselves. 
In 1885 Mr. J. S. Kingsley associated the Limulus with the Arachnids as a group by them- 
selves, to which he gave the name Acera (Science News and Quart. Journ. Micr. Se.). 
In 1886, in the 5th edition of our Text-book of Zoology, we suggested the term Podostomata 
for the class comprising the two orders Merostomata and Trilobita. 
TV.—The class Podostomata. 
It thus appears that while at the present date (1886) A. Milne Edwards, E.Van Beneden, and 
B. R. Laneaster regard Limulus and its allied forms as belonging to the Arachnida, and J. S. 
Kingsley associates the Limulus and the Arachnida in a group by themselves under the name 
Acera, the present writer and Professor Claus regard the Merostomata with the Trilobites as 
forming a class intermediate between the Arachnida and Crustacea. 
We have endeavored to show that the names Pecilopoda and Gigantostracea have been applied 
in such different senses by different authors that they cannot well be retained for the Merostomata 
and Trilobita taken together in the sense we advocate. We have therefore proposed the term 
Podostomata for this class of Arthropoda. Itis derived from zovs, zod0's, foot, and oro a, mouth, 
in allusion to the foot-like or ambulatory nature of the cephalic appendages which surround the 
mouth in a manner characteristic of the group. , 
The class Podostomata may be defined as a group of Arthropods, in which the cephalic 
(Limulus) or cephalo-thoracic (Trilobites) appendages are in the form of legs, %. ¢., ambulatory 
appendages, usually ending in forceps, or large claws (chele), which in the sole living representa- 
tive of the class are arranged in an incomplete circle around the mouth; the basal joint of each 
leg is spiny, so as to aid in the retention and partial mastication of the food. No functional 
antenne, mandibles, or maxilla. Eyes both compound and simple. Respiration by branchiz 
attached to the abdominal appendages, which are broad and lamellate in Merostomata and probably 
cylindrical with narrow gills in Trilobita. The brain (procerebrum) supplying nerves to the eyes 
alone; the nerves to the cephalic or cephalo-thoracic appendages originating from an esophageal 
ring; the ventral cord ensheathed by a ventral arterial system more pertectly developed than in 
insects or scorpions; coxal glands highly developed, with no external opening in the adult. This 
class differs from the Arachnida, among other characters, in having no functional cheliceres (*‘ man- 
dibles”) or pedipalps (‘‘maxille”); in the cephalic appendages either ending in large claws or 
forceps, or simple, the terminal joint not bearing a pair of minute claws or ungues like those of 
Arachnida and Insecta, enabling their possessors to climb as well as walk. Podostomata have no 
urinary tubes. Limulus undergoes a slight metamorphosis, while in Trilobites the adult differs 
from the larva in having a greater number of thoracic segments. 
