156 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



In 1885 and 1886 (Annals and Mag. Nat. Hist., July, 1880) Glaus regarded the Gigantostraca 

 as a class intermediate between the Crustacea and Arachnida. He thinks that ttie Arachuida 

 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 aad Palceocarida, 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.Walcott (Bull. Mus. Comp. Zooi., viii. No. 90, p. 209), under the class 

 Pcecilopoda, places two subclasses, viz, Merostomata and Palseadfe (Trilobita), giving definitions 

 of the groups. 



In 1881, in his article "Limulus an Arachnid" (Quart. .Tourn. Micr. Sc.) Prof. E. Ray Lan- 

 kester proposed the term Hcematobranchia, 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, III, 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. Sc). 



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. 



IV. — The cluss Podostomata. 



It thus appears that while at the present date (1886) A. Milne Edwards, E.Van Beneden, and 

 E. E. Lancaster 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 Arachuida and Crustacea. 



We have endeavored to show that the names Pcecilopoda 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. It is derived from nov?, nodo'?, foot, and aro na, 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, i. e., ambulatory 

 appendages, usually ending in forceps, or large claws (chela), 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 

 antennae, mandibles, or maxillae. Eyes both compound and simple. Respiration by branchiae 

 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 oesophageal 

 ring; the ventral cord ensheathed by a ventral arterial system more perfectly 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 ("maxilhe"); 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. 



