450 THE CH^ETOGNATHA. 



a cephalic navel. The body and the alimentary canal are straight, that is, there 

 is no marked neural or haemal curvature, and no multiplication by budding, agree- 

 ing in these respects with the cephalochorda and the enteropneusta. 



The trunk of the adult is elongated, spindle-shaped, flattened on the neural 

 and rounded on the haemal surface, with broad pleural, or thoracic folds, of various 

 forms in different genera. The caudal portion is provided with a telson-like 

 terminal lobe. 



The head of the adult undergoes but little change over that seen in the young 

 larva, the most conspicuous features being the prepuce-like mantle folds, represent- 

 ing the bivalve shell of the nauplius, pr, and the pair of large muscular lobes or 

 mandibular-like appendages, md. They are provided with a group of sensory, 

 or glandular follicles, /<?., and armed with stout movable spines consisting of a 

 chitenoid material, apparently perforated with typical pore canals and enclosing 

 a conspicuous pulp cavity. The structure of the follicles and spines gives the 

 appendages a decidedly arthropod appearance. 



The Endocranium. The mandibles and their spines are moved by power- 

 ful adductor muscles, in which is imbedded what appears to be a small median 

 fibroid, or cartilaginous plate, en.c. 



It was this plate that Grassi referred to as an organ of unknown significance, 

 but which might possibly prove to be a "precious jewel" in the eye of the mor- 

 phologist. 



In his description of the muscles in question, he states (p. 42.): "Esso e 

 dentro 1'involucro chitinoide del complesso mediano. Questo musculo, siccome 

 dissi, ha forma d'arco concavo anteriormente; in un suo-seno, sotto al punto di 

 massima concavita delParco, riposa un corpicciolo ovale, appiattito nel senso 

 dorso-ventrale, fatto di cellule neucleate piuttosto piccole ed a contorni piu or 

 meno decisi; esso e coperto di fibre muscolari da ogni lato, eccetto il dorsale 

 e 1'anteriore, dov'e separate dalP esofago per 1'involucro chitinoide del musculo. 

 Questo involucre e la musculatura lo separano dalla commissura nervosa boccale. 

 Ho riscontrato 1' organo in discorso in tutte le specie, eccetto la Claparedi; piu 

 voluminose sono le specie, piu compare grosso. Per quanto indagassi non riuscii 

 ad intenderne la significazione fisiologica e morphologica. Posso congetturare 

 soltanto che si tratti di un organo o nascente o rudimentale, e percio di una pietra, 

 forse preciosa agli occhi del morphologo." 



The plate in question no doubt represents a small endocranial cartilage, or 

 sinew, serving for the attachment of the mandibular muscles. It has a special 

 significance for us because we have seen that a similar cartilage is imbedded in the 

 mandibular muscles of many primitive arthropods, as in Branchipus, Apus, and 

 many others, and that it forms the starting point for the cartilaginous endocra- 

 nium of the higher arthropods and of the vertebrates. See Chapter XVII, p. 312. 



Nervous System. The chaetognaths are the only acraniates that retain in the 

 adult a comparatively large forebrain, with the full equipment of stomodaaal 

 ganglia, nerves and cephalic sense organs, B, f.br. 



