No. I.] STUDIES ON LIMC/LUS. 13 



the base of the operculum and is indented by two pits [op.m.), 

 which represent the origin of a pair of muscles inserted on the 

 inside of the appendage. 



The posterior and anterior prolongations of the endochondrite 

 (/./. and a.p., Fig. 3) serve partly for the attachment of muscle 

 strands of the longitudinal muscles of the abdomen, but in 

 many cases the anterior and posterior processes of successive 

 endochondrites are continuous with each other. On the haemal 

 side of the endochondrite is a pair of haemal processes {h.p., 

 Fig- 3), one on each side of the ventral cord. These project 

 haemally, and a little outward and backward, and furnish 

 attachment for a pair of haemo-neural muscles inserted on the 

 haemal side of the carapace just median to the entapophyses. 



III. The Endosternite of Apus. 



(PI. II, Figs. 5-10.) 



This structure, like that of Limulus, is located in the cephalo- 

 thorax, between the central nervous system and the intestine. 

 It lies behind the mouth and opposite the mandibles. The body 

 of the plastron is elongated in a direction transverse to the long 

 axis of the animal, and its flaring ends (w.. Figs. 5, 6, and 

 10) give attachment to the powerful adductor muscles of the 

 mandibles. 



On the posterior side a pair of chitinous apodemes («/<?., 

 Figs. 5, 6, 8-10) project into the plastron. These are formed 

 by the invagination of the chitin between the bases of the first 

 and second appendages behind the mandibles. These append- 

 ages have been called the first and second maxillae. ^ Lankester 

 regards them as one appendage consisting of two parts and 

 calls it a maxilla. Bernard calls the anterior portion a cleft 

 underlip, and the posterior portion the first maxilla. 



From the inner ends of the apodemes a pair of tendonous 

 cords run forward directly through the body of the plastron at 

 right angles to its fibers, and emerge on the anterior side as a 

 pair of anterior cornua {a.c, Figs. 5, 6, 8, and 9). These proc- 



1 A. Gerstaecker, Die Klassen unci Ordmaigen der Arthropodett, Bd. v. Crustacea. 



