DERIVATION OF THE MESOBLAST. 34! 



from the alimentary cavity form the water- vascular system and 

 the somatic and splanchnic layers of mesoblast. The cavity of 

 the diverticula after the separation of the water-vascular system, 

 forms the body-cavity. The outer lining layer of the cavity forms 

 the somatic layer of mesoblast and the voluntary muscles-; the 

 inner lining layer the splanchnic mesoblast which unites with the 

 epWtelium of the alimentary tract. Though this fundamental 

 arrangement would seem to be universal amongst Echinoderms, 

 considerable variations of it are exhibited in different groups. 



There is one outgrowth from the alimentary tract in Sy- 

 napta; two in Echinoids, Asteroids and Ophiura; three in 

 Comatula, and four (?) in Amphiura. The cavity of the out- 

 growth usually forms the body-cavity, but sometimes in Ophiura 

 and Amphiura (Metschnikoff) the outgrowths are from the first 

 or soon become solid, and only secondarily acquire a cavity, 

 which is however homologous with the body-cavity of the other 

 groups. 



In Sagitta 1 the formation of the mesoblast and the ali- 

 mentary tract takes place in nearly the same fashion as in the 

 Echinoderms. The simple invaginate alimentary cavity becomes 

 divided into three lobes, a central and two lateral. The two 

 lateral lobes are gradually more and more constricted off from 

 the central one, and become eventually quite separated from it ; 

 their cavities remain independent, and form in the adult the 

 body-cavity, divided by a mesentery into two distinct lateral 

 sections. The inner layer of each of the two lateral lobes forms 

 the mesoblast of the splanchnopleure, the outer layer tfie mesoblast 

 of the somatopleure. The central division of the primitive 

 gastraea cavity remains as the alimentary tract of the adult. 



The remarkable observations of Kowalevsky* on the devel- 

 opment of the Brachiopoda have brought to light the unexpected 

 fact that in two genera at least (Argiope and Terebratula) the 

 mesoblast and body-cavity develope as paired constrictions from 



from the hypoblast, and that the diverticula only form the water-vascular system and 

 the epithelial lining of the body-cavity. 



1 Kowalevsky, " Wiirmer u. Arthropoden," Mem. Acad. Petersbourg, 1871. 



* "Zur Entwicklungsgeschichte d. Brachiopoden ", Protokoll d. ersten Session der 

 Versammlung Russischer Naturforscher in Kasan, 1873. Published in Kaiserliche 

 Gesellschaft Moskau, 1874 (Russian). Abstracted in Hoffmann and Schwalbe, Jahres- 

 btricht f. 1873. 



