ON THE ORIGIN OF VEETEBEATES FEOM AEACHNIDS. 353 



short and massive, but it finally extends forwards (partly, per- 

 haps, by an actual forward growth, but mainly owing to its 

 being left behind by the backward growth of the posterior end 

 of the body) the whole length of the abdomen as a rather large 

 cylindrical cord; the latter, about the time of hatching, splits 

 into two parts, an outer one composed of a thin layer of small 

 dark nuclei, representing the "anlage'^ of the botryoidal 

 cord, and an inner one composed of large polygonal cells, repre- 

 senting the anlage of the sexual organs (Fig. 2, c). 



The anlage of the botryoidal cord soon develops, at irregu- 

 lar intervals, spindle-shaped enlargements, each intermediate 

 portion being reduced to a delicate hyaline fibre. The spindles, 

 eight to ten in number, become attached to the wall of the 

 spinal artery, and in most cases an imperfect communication 

 is established between the cavity of the artery and the interior 

 of the spindle (Fig. 2, e, also d. bt. c). 



In the adult the spindles are composed of botryoidal masses 

 of fibroid tissue densely packed with small, deeply stained 

 nuclei. The organ is, perhaps, a gland for the production of 

 blood-corpuscles. 



If the spinal artery of Scorpio represents a notochord, then 

 the botryoidal cord probably represents the " subchordal rod." 



IX. The Cartilaginous Sternum and the Primordial 

 Cranium. — A characteristic organ of the Arachnida is the 

 cartilaginous sternum or " endocranium." In Scorpio 

 (and in Limulus it is about the same) it is a broad, lyre- 

 shaped bit of fibroid cartilage, with forwardly directed arms, 

 lying beneath the hind-brain (Fig. 12). Its posterior portion 

 is more massive, and completely surrounds the posterior 

 portion of the brain, forming a kind of occipital cartilaginous 

 ring (Fig. 2, r). It develops from the mesoderm as a mem- 

 branous diaphragm underlying the thoracic neuromeres. In 

 size, shape, position, structure, and manner of development — 

 in fact, in every particular except its chemical composition, the 

 endocranium of Scorpio corresponds with the primordial 

 cranium of Vertebrates. 



