12 Alexander Petrnnkevitch, 



The second class or Araclmida are characterized by Lankester as 

 follows : 



"Head diprosthomerous, tritognathous — that is to say, with two 

 })rost homer es, the first bearing typical eyes, the second a pair of 

 appendages reduced to a single ramus which is in more primitive forms 

 antenniform, in higher forms chelate or retrovert. The ancestral 

 stock was pantognathobasic — i. e. had a gnathobase or jaw process on 

 every parapodium. As many as six pairs of appendages following 

 the mouth may have an enlarged gnathobase actually functional as 

 a jaw or hemignath, but a ramus is well developed on each of these 

 appendages either as a simple walking leg, a palp or a chela. In the 

 more primitive forms the appendage of every post-oral somite has a 

 gnathobase and two rami ; in higher specialized forms the gnatho- 

 bases may be atrophied in every appendage, even in the first post- 

 oral. 



"The more primitive forms are anomomeristic ; the higher forms 

 nomomeristic, showing typically three groups or tagmata of six 

 somites each. 



"The genital apertures are placed on the first somite of the second 

 tagma or mesosoma. Their position is unknown in the more primi- 

 tive forms. The more primitive forms have branchial respiratory pro- 

 cesses developed on a ramus of each of the postoral appendages. 

 In higher specialized forms these branchial processes become first 

 of all limited to five segments of the mesosoma, then sunk beneath 

 the surface as pulmonary oigans, and finally atrophied, their place 

 being taken by a well-developed tracheal system. 



"A character of great diagnostic value in the more primitive Arach- 

 nida is the tendency of the chitinous investment of the tergal surface 

 of the telson to unite during growth with that of the free somites 

 in front of it, so as to form a pygidial shield or posterior carapace, 

 often comprising as many as fifteen somites (Trilobites, Limulus)." 



Sub-divisions of the class Arachnida according to Lankester: 



Class ARACHNIDA 

 Grade A. Anomomeristica 

 Sub-class Trilobitae 



Orders. Not satisfactorily determined 

 Grade B. Nomomeristica 

 Sub-class I. Pantopoda 



Order i. Nymphonomorpha 

 2. Ascorhynchomorpha 

 ,, 3. Pycnogonomorpha 



