NERVOUS SYSTEM OF SCORPION. 45 



Stage 2. - - (Fig. 2.) Seventeen segments have formed. Ap- 

 pendages are present on segments two to fourteen and neuro- 

 meres in segments one to fourteen. The median furrow is con- 

 tinued backward and is very broad in segments twelve to 

 fourteen, a condition probably due to the pressure of the post 

 abdomen, which is turned under the pre-abdomen during the 

 development of the embryo and not straightened out as shown 

 in the figure. In addition to the numerous pits two pairs of 

 invaginations have appeared in the first neuromere (ci l , ci 2 ). 

 These invaginations, while larger than the pits, are probably 

 similar to them physiologically, for like them they are only tem- 

 porary structures, being finally filled up by the growth of the 

 cells forming their walls. Pits like those in the neuromeres have 

 formed, externally and internally at the bases of the thoracic 

 appendages and probably represent sense organs, as Patten main- 

 tains. It is not probable however that the pits in the neuromeres 

 represent sense organs, since the embryology of the scorpion is 

 of so specialized a type that we would not expect to find such 

 remotely ancestral structures repeated here, when not repeated 

 in more generalized types. The mouth has begun to move back- 

 ward. This is accomplished in two ways : First there is an actual 

 displacement, the ectoderm surrounding the mouth being thin 

 and probably offering little resistance ; second, the mouth opening 

 elongates and the anterior portion closes by the growing together 

 and fusing of the two sides, the remaining opening .being further 

 back than the center of the original opening. The closure of the 

 anterior portion of the mouth forms a lip called the rostrum 

 (Fig. 3, r). It is thought by some that the rostrum represents 

 the fusion of a pair of appendages. That it is formed by fusion 

 is clear, but the only evidence I see in favor of regarding it as 

 derived from appendages is the fact that in the later embryo and 

 the adult it is innervated by a special nerve (Fig. 8, R}. This 

 nerve innervates the muscles of the anterior part of the cesoph- 

 agus, or pharynx, also and might as well be called a pharyngeal 

 as a rostral nerve. 



Stage j. (Fig. 3.) Twenty segments, the complete number, 

 have formed, and neuromeres are present in all of them. In the 

 anterior part of each of segments three to twelve an elliptical 



