xviii HISTORICAL SKETCH. 



living representatives of the now extinct merostomes, or giant sea scorpions, that 

 were regarded as the arachnids standing nearest to the ostracoderms. 



Other evidence and conclusions were as follows : i . In the arachnids a f orebrain 

 vesicle is formed by the same process of marginal overgrowth as in the vertebrates. 

 From the floor of the vesicle arise the f orebrain and optic ganglia; from the 

 membranous roof, a tubular outgrowth is formed that contains a parietal, or pineal 

 eye, similar in structure, mode of origin, and innervation to the pineal eye of 

 vertebrates. 2. The kidney-shaped compound eye of arachnids has been trans- 

 ferred to the walls of the cerebral vesicle in vertebrates, giving rise to the retina, 

 which still shows traces of ommatidia in the arrangement of the rod-and-cone 

 cells. Its original shape is temporarily retained in vertebrates, but gives rise 

 ultimately, by adaptive exaggeration, to the choroid fissure. 3. The arachnids 

 have a cartilaginous endocranium similar in shape and location to the primordial 

 cranium of vertebrates. 4. They have an axial, subneural rod comparable with 

 the notochord. 5. In arachnids, the brain contains approximately the same num- 

 ber of neuromeres as in vertebrates. It is also divided into similar regions, each 

 one having a similar number of neuromeres, a similar distribution of nerves, and a 

 similar relation to cranial ganglia and sense organs, to those in vertebrates. 6. 

 The segmental sense organs (median and lateral eyes, olfactory and auditory 

 organs) are comparable with those in vertebrates. The coxal sense organs are 

 associated with special sensory nerves and ganglia, comparable with the cranial 

 dorsal-root nerves and ganglia (suprabranchial sense organs) of vertebrates. 

 7. The basal arches of the appendages are comparable with the oral and branchial 

 visceral arches in vertebrates. 8. The tendency toward concentration of neuro- 

 meres has narrowed the passage way for the stomodeum and modified the mode of 

 life in the arachnids. This ultimately led to its permanent closure, the infundi- 

 bulum and adjacent nerve tissues in vertebrates representing the remnants of 

 the old stomodagum with its nerves and ganglia. 10. The progressive degeneration 

 of haemal thoracic muscles, the fusion of thoracic metameres, the position of the 

 oral, or neural surface, in swimming and crawling, were identified with corre- 

 sponding conditions in vertebrates. 11. The eye muscles of vertebrates arose 

 from a special group of hsemo-neural muscles belonging probably to the first 

 two or three thoracic segments. 12. The process of gastrulation in vertebrates 

 and arachnids is confined to the procephalic lobes, in the place where at a later 

 period the primitive stomodaeum appears. The so-called "gastrulation" of verte- 

 brates and arachnids is an entirely different and independent process, that is, the 

 process of adding by apical or teloblastic growth a segmented, bilaterally sym- 

 metrical body to a primitive radially symmetrical head. 13. The arachnids 

 resemble the vertebrates in more general ways, as in the minute structure of 

 cartilage, muscle, nerves, digestive, and sexual organs. 



In the following paper, '93, the structure of the forebrain of Limulus, with its 

 lobes and cavities was compared in detail with the brain of vertebrates. The 

 coxal sense organs were described and shown to be gustatory organs comparable 



