ZOOLOGICAL POSITION OF LIMULUS. 223 



and Pterygotus, and also the large compound eyes and the respiratory system in both, 

 differ entirely from any structure to be found amongst existing forms of Arachnida. I 

 long held the opinion that the organs called combs in the Scorpion were homologous 

 with the branchial leaves of Pterygotus : they are, so far as I am aware, aborted organs 

 in Scorpio, although they are supposed to fulfil some excitatory function in connection 

 with the impregnation of the ovaries : may they not probably be homologous with 

 the opercular plates of Limulus covering the branchial plates, and bearing the orifices 

 of the reproductive organs on their inner surface ? 



As I have elsewhere stated, there is no insuperable difficulty in accepting, on sound 

 physiological grounds, the possibility of any animal passing through larval conditions, 

 casting aside at even a single moult its branchiae, and assuming aerial respiration, 

 quitting the water and inhabiting the land, changing its element, its diet, its mode of 

 progression, and its entire life. Such cases are familiar to the entomologist, 1 thecarcino- 

 logist, 2 and even to the herpetologist. 3 



From his examination of the embryonal changes of Limulus Dr. Dohrn has been led 

 to observe how remarkably the young stages agree with the youthful Sao hirsuta and 

 Trinucleus ornatus ; indeed, he might well add, with the young of Agnostus, Aulaco- 

 pleura, Phacops, Hydrocephalus, and with every Trilobite of whose infancy the 

 researches of the illustrious Barrande has made us acquainted (see PI. XXXII). 



He also notices the remarkable analogy which the nearly mature embryo presents to 

 Bettinurus and Presttoichia — a resemblance which I had previously pointed out in my 

 paper already referred to (' Quart. Journ. Geol. Soc./ 1860, vol. xxiii, pp. 34 — 35). 



I will now quote the final paragraph from Dr. Dohrn's paper, and then give my 

 reasons for dissenting from his conclusions. 



" The more the theory of evolution is allowed a direct influence in zoological work, 

 the more conspicuous is its pre-eminently practical character, inasmuch as it not only 

 solves problems, but also suggests a correct method of inquiry. Thus, it is true, it 

 destroys the traditional system ; but it sets up a new one immediately ; and thus, if we 

 ask for the result of its application as made in the present memoir, we arrive at the 

 following conclusions : 



" Limulus is most nearly related to the Gigantostraca ; both seem to be related to 

 the Trilobites, although it is not possible to demonstrate all the details of their relation- 

 ship. The morphologico-genealogical relations of these three families to the Crustacea 



1 Larval and adult Libellulce, Ephemera, &c. 

 3 Gecarcinus ruricola and other Land-crabs. 



3 The Batrachia. 



4 Prof. Hackel believes the Trilobites to be a suborder of the Phyllopoda, from which, perhaps, the 

 Pcecilopoda descended. He again divides the Pcecilopoda into two legions, viz. the Xiphosura and 

 Gigantostraca. (See his ' Generelle Morphologie,' the Gigantostraca being Ptenjgotus, Eurypterus, &c.) 

 In this view Gegenbaur agrees with Hackel. 



