Note on the Legs of Trilobites. 63 



hand. Future discoveries will undoubtedly add to and change them some- 

 what — these notes being intended to show the progress made up to the present 

 time, and not as a final publication. 



First. The Trilobite had a thin ventral membrane beneath the visceral 

 cavity, strengthened by arches, which supported the appendages beneath. The 

 membrane extended outward from the main visceral cavity, and joined the edge 

 of the doublure of the head, thoracic segments and pygidium, somewhat as the 

 sternum of Limulus is connected to the margins of its shell by a membraneous 

 crust. 



Second. Attached to the ventral surface, on a line with the outer edges of the 

 alimentary canal, there is a row of articulated appendages on each side — the 

 leg consisting of five or more joints, the terminal joint being provided with a 

 claw, and the basal joint with a point of attachment for a jointed arm. 



Third. The jointed appendage attached to the basal joint of the leg is homolo- 

 gous with the epipodite of recent crustaceans. (This statement is made with 

 some reserve, as the evidence is not positive. The basal joint, as seen in all 

 the sections showing it distinctly, is large and articulated «to the ventral surface. 

 Should this basal joint ultimately prove to be formed of two joints — too closely 

 segmented to be separated in the present sections, and the arm attached to the 

 second joint — the homology will be with the expodite. And, as it is a gill- 

 bearing organ, it may be homologized with that organ in Mysis.) 



Fourth. The respiratory apparatus consists of a gill-bearing appendage 

 attached to the thoracic leg and a bifed spiral gill attached to the side of the 

 thoracic cavity. The setiferous appendages attached to, or above, the manduca- 

 tory legs are modified thoracic branchiae. 



Fifth. The mouth is posterior to the hypostoma, and consists of the four pairs 

 of manducatory jaws, formed by the basal joints of the smaller appendages and 

 the larger pair of the posterior pair of appendages. (An oblique section of what 

 may prove to be the swimming-appendage was found in a section of the same species 

 of Trilobite, from which the parts of the mouth have been obtained. It has 

 three slender joints attached. A small spine projects from one towards the 

 terminal joint. It was probably attached to one of the posterior manducatory 

 legs as a portion of the swimming apparatus of the Trilobite.) 



Sixth. Admitting the third conclusion to be correct, in regard to the 

 attachment of the jointed arm to the basal joint, we can homologize the leg and 

 epipodite with that of the thoracic leg of the larval lobster. Its relation to 

 Mysis, in the event of its being the expodite, is mentioned in the third conclusion. 

 The homology between the parts about the mouth of the Trilobite and the same 

 organs in the Eurypterida and Zyphosura is very direct, and relates the families 

 closely. In view of these relations, the following arrangement is made. Taking 

 Prof. Henry Woodward's classification of Zyphosura and Eurypterida under 

 Gnathopoda, and adding the Trilobita, we have : 



Class— CRUSTACEA. 



Subclass— GNATHOPODA. 

 Legion — Mbrostomata. 

 Order 1 — Zypttosura. 

 " 2 — Eurypterida. 

 " 3 -Trilobita. 



