AND EMYRYOLOGY OP LIMULUS. 7 



all breathing by tracheae, excepting the few species which have no breathing organs at all, 

 that it seems most advisable to retain them as sub-divisions or sub-classes of the class of 

 insects or Tracheata. 



There is little in common between the mouth-parts of Limulus and those of the Arach- 

 nida, either in their form or grouping ; moreover, the mouth-parts of Limulus are not 

 differentiated from the other cephalothoracic appendages. The six pairs are alike ; 

 morphologically true gnathopods ; and in the embryo arise simultaneously ; in the Arach- 

 nida, the two pairs of mouth-parts are, in adult life, quite different from the eight legs, 

 and are soon differentiated in early embryonic life. Limulus resembles the Arachnida 

 in the want of antennae, but so important are the differences in the mouth-organs and legs, 

 that it seems a violation of the principles of classification to associate together the two 

 types within the limits of the same class. 



The second Arachidan feature claimed by authors to exist in Limulus is the alleged 

 similiarity in the form of the nervous system to that of the Arachnida, especially the 

 scorpions and spiders. The oesophageal collar of the horse-shoe crab has been homolo- 

 gized with the thoracic ganglionic mass of Arachnida, and the brain of Limulus has been 

 likened to that of the spiders and of the scorpions. 



The brain of Arachnida has heretofore been supposed to be a single pair of ganglia, and 

 to send nerves not only to the simple eyes, but also to the first pair of mouth appendages. 

 If this view is correct, as all who have studied the adult Arachnids agree, then the brain of 

 Limulus is not homologous with the arachnid brain (supra-oesophageal ganglion), as it sup- 

 plies only the eyes, sending no nerves to the anterior gnathopods. As will be seen farther 

 on (Plate 4, fig. 7 gn), the first pair of gnathopods is supplied in the larva directly from 

 an independent pair of ganglia. Very recently, however, Mr. Balfour ^ has proved that 

 the so-called supra-oesophageal ganglion or brain of the spider is formed of two pairs of 

 ganglia which at first are quite distinct, as shown by his section of the embryo spider. 

 Mr. Balfour concludes that " the evidence which I have got that the cheliceres are true 

 postoral appendages, supplied in the embryo from a distinct postoral ganglion, confirms the 

 conclusions of most previous investigators, and shows that these appendages are equiv- 

 alent to the mandibles, or possibly the first pair of maxiUae of other Tracheata." 



Li either case then, whether the brain of Arachnida is a single pair of ganglia, sup- 

 plying the chehceres (or mandibles), as well as the ocelli or two pairs of consolidated 

 ganglia, the brain of these Arthropods can scarcely be homologous with the brain of 

 Limulus. 



Moreover, the position of the brain in relation to the thoracic ganglionic mass of Arach- 

 nida is quite difierent from that of Limulus ; in the former animals, judging fii-om Blanch- 

 ard's beautiful and accurate plates, and our own examination of the brain of the scorpion, 

 it is invariably situated in a plane parallel to and much above the thoracic mass, and 

 separated by long slender commissures ; while the brain of Limulus is situated on the 

 same plane as the oesophageal collar, in fact, closing up the front of what would otherwise 

 be an open ring or collar. 



1 Notes on the development of the Araneina. By F. M_ April, 1880, pp. 176, 185, 189, PI. xxi., fig. 21. 

 Balfour. Quarterly Journal of Microscopical Science, 



