320 WILLIAM PATTEN. 



frequent presence in Arthropods, especially Crustacea, of 

 bifurcated appendages ; this condition is due, we may siippose, 

 to the partial fusion of two originally distinct appendages. 

 (6) The frequent occurrence of insect monsters having double 

 pairs of legs. (7) According to Heathcote's important obser- 

 vations, the segments in Julus are certainly double, as shown 

 by the duplication in each segment of the somites, cardiac ostia, 

 arteries, ueuromeres, tracheae, and legs. (8) In Scorpio the 

 neuromeres are distinctly double, each one being composed of 

 a large anterior portion and a small posterior one. Large pit- 

 like invaginations of the median furrow are found between 

 the halves of the anterior portions, and faint indications of a 

 second series of pits between the halves of the posterior por- 

 tions (PI. XXIV, fig. 3). But in Scorpio the most singular 

 feature of all is that the parts of each abdominal neuro- 

 mere finally separate, the posterior portions uniting with 

 the anterior portion of the neuromere just behind it (PI. 

 XXIV, figs. 3 and 4, and Fig. 11, p. 348). This process 

 may be followed with ease and perfect certainty in surface 

 views. 



All these facts point to the conclusion that the segments in 

 all Arthropods are double, and are derived from those of 

 diplopod-like ancestors. 



If Vertebrates are derived from Arthropods, they are also, 

 in all probability, composed of double segments. It may be 

 worth mentioning in this connection that in many fishes the 

 spinal nerves, and especially the cervical ones, split up into 

 two, three, and sometimes four pairs of nerves for each neuro- 

 mere. 



I. The Grouping of the Cranial Neuromeres of Scor- 

 pio is a result of the varying union of the first thirteen 

 neuromeres. 



From the cephalic lobes three neuromeres arise which fuse 

 completely to form the fore-brain of the adult (figs. 1—4, 

 Pis. XXIII and XXIV, and Fig. 1, f. b., p. 321). 



The first neuromere of the six thoracic segments pushes its 



