The Anatomy, Hjstologt and Ejibryologt of Limulus Polyphemus. 



By a. S. Packard, Jr., M.D. 



loINCE the publication of my first paj^er on the development of the horse-shoe or 

 king crab {Limulus polyphemus), in the Memoirs of this Society/ I have, as opportunity 

 allowed, made additional observations on the development of the larva, and also 

 on the histology of the different organs, and especially the brain. In making the 

 microscopic sections of the embryos and for a series of sections of a brain, the 

 latter of which were unstained, I am indebted to Professor T. D. Biscoe. For mounting: 

 some of these sections for study, I am indebted to Dr. C. B. Johnson of Providence, R. I., 

 who also kindly cut, stained, and mounted jjreparations of the digestive canal. Within 

 the past year I again returned to a study of the brain, using the methods of staining 

 employed by German observers, Dietl and Krieger, also by Mr. E. T. Newton. The 

 sections of the brain were cut and stained, as also those of the eyes, parts of the 

 stomach and rectum, kidneys and liver, etc., by Mr. Norman N. Mason, of Providence, 

 R. I., who kindly devoted a great deal of time to the work. To his unusual skill and 

 delicacy of manipulation, I am indebted for a large number of preparations much better 

 than I could have made myself, and which have been of most essential aid in preparing 

 this paper ; so that portions of the histological part of this paper, especially that 

 on the structure of the eyes, are really joint productions with Mr. Mason, as we together 

 examined the pi'eparations. 



Position of Limulus among Arthropoda. 



The researches of M. Alphonse Milne-Edwards on the anatomy of Limulus, proved 

 that this animal, so far from being a genuine normal crustacean, is either the type 

 of a group equal to all the other Crustacea, namely a sub-class of Branchiata ; or, as 

 several authors contend, should be regarded as the representative of a distinct class of 

 Arthropoda. 



Before arguing what we now believe to be the true position of Limulus and 

 the allied fossil forms, including the Trilobites, let us take a review of the different 

 opinions of the leading zoologists who have done special work on the animal. The 

 titles of their work will be found in the bibliographical list at the end of this paper. 



' Memoirs Bost. Soc. Nat. Hist., ii, 155-202. 



