No. I.] STUDIES ON LIMULUS. 5 



Methods. 



In the work upon Linmlus both fresh and alcoholic material 

 was employed, but in that upon Mygale and Apns only alcoholic 

 specimens were used. The plastron of Limidiis, with adjoin- 

 ing parts, was excised and allowed to soften in water until the 

 muscles could be easily picked away. Dissection was then car- 

 ried on under water or weak alcohol with fine pointed forceps. 

 In other specimens the muscles were allowed to macerate 

 completely, and were then brushed off, leaving the cartilage 

 intact. 



Serial sections of Apus, both transverse and longitudinal, 

 were made, and a model of the plastron magnified seventy times 

 was reconstructed from the longitudinal sections. 



As Mygale material was scanty, only cross-sections of this 

 animal were made. The viscera were cut from the cephalothorax 

 and imbedded in celloidin. Sections were cut and mounted 

 serially, stained in borax carmine, and a reconstruction of the 

 cartilage made with an amplification of twenty times. With 

 these models as guides, drawings were made and minute details 

 added by careful study of the sections under the microscope. 



I. Plastron of Limulus. 



(See PI. I, Figs. 1-4.) 



I. General Description. — In the following descriptions, for 

 reasons which will appear later, the terms " haemal " and 

 " neural " will be substituted for dorsal and ventral, respec- 

 tively, ajid the terminology of Lankester will be used in desig- 

 nating the parts of the endosternite. 



In Liniuhis the plastron lies in about the center of the 

 cephalothorax, with the anterior margin about opposite the 

 chelicerae, and the posterior extremity on a level with the chi- 

 laria. The anterior processes extend some distance beyond the 

 bases of the chelicerae. The mouth of the animal is located a 

 little anterior to the center of the plastron, and from it the 

 oesophagus passes forward between the anterior processes to 

 form the <;-shaped proventriculus, the haemal arm of which 



