lxxii PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [Ma 1 895, 



a monograph published by the Palaeontographical Society (1865- 

 1878, pp. 263, pi. xxxvi.), comprising 17 genera and 84 species — 

 69 of which are Palaeozoic in age. Writing to me, in ~Nov. 1867, 

 Prof. Huxley says : ' I am a convert to the Merostomata in your 

 sense.' 



The integrity of this group, founded on the researches of Huxley, 

 Salter, Dana, Hall, and many others besides myself, has been 

 maintained, although many attempts have since been made to 

 detach it from the Crustacea and place it with the arachnida. 

 For instance, it was proposed by Dr. Dohrn, in 1871, 1 to include 

 the merostomata in a still larger division, under Haeckel's term 

 Gigantostraca, which was made by expansion to embrace the 

 merostomata and the trilobita, and to be placed between the Crus- 

 tacea and the arachnida. 



Against this classification I urged that these animals, more espe- 

 cially the trilobita, being certainly amongst the earliest forms of 

 Crustacea with which we are acquainted, cannot be removed from that 

 class without destroying its ancestral record. In arguing for their 

 retention before this Society in 1871 (Quart. Journ. Geol. Soc, 

 vol. xxviii. pp. 46-62) I wrote : — ' Take away the trilobita from 

 the pedigree of the Crustacea, and I submit that one of the main 

 arguments in favour of evolution to be derived from the class, so 

 far from being strengthened, is destroyed. From what are the 

 Crustacea of to-day derived? Are we to assume that they all 

 descended from the phyllopods and ostracods — the only two re- 

 maining orders whose life-history is conterminous with that of the 

 trilobita ? Or are we to assume that the arachnida are the older 

 class ? ' 'If (says Fritz Miiller) ' the Crustacea, insecta, myria- 

 poda, and arachnida are indeed all branches of a common stock, it 

 is evident that the water-inhabiting and water-breathing Crustacea 

 must be regarded as the original stem from which the other terres- 

 trial classes, with their tracheal respiration, have branched off.' 



In the above-quoted paper (op. cit. p. 53) I pointed out that the 

 young Limulus, when it quits the egg (at its 10th stage), has the 

 hinder body as large as the head-shield, and the nine segments 

 composing it are most clearly marked out, the abdominal spine 

 being quite rudimentary and forming in fact the 9th segment. 

 This is the so-called ' Trilobiten-stadium ' (Trilobite-stage) of 

 Dohrn. 



1 ' Zur Einbryologie unci Morpbologie des Limulus polyphemus] von Dr. Anton 

 Dohrn, Jenaische Zeitschrift, vol. vi. Heft 4, p. 580, pis. xit. & xv. 



