CH^TOGNATHA 871 



dorsal and a ventral mesentery are formed by their contiguous walls 

 (Fig. 163 iiifis). The young Sagitta upon hatching from the egg exhi- 

 bits essentially the form of the adult animal. 



General Considerations. — The problem as to the position of the 

 Chffitognatha in the zoological system is not brought any nearer to 

 solution even by embryology, and for the time being can be treated 

 only with the utmost reserve. The agreement which exists between 

 the transverse section through Sagitta and that through Polygordius 

 has already been pointed out by O. Hertwig. As a matter of fact, 

 a significant resemblance in the tectonic of the two groups is shown in 

 the presence of paired entodermal sacs lined with epithelium, a dorsal 

 and ventral intestinal mesentery, and the four longitudinal muscle- 

 bands arranged in pinnate lamellffi, to which in some cases the 

 indication of a transverse musculature is added. The chief difficulty 

 in arriving at a safe conclusion regarding the position of the Chcetog- 

 natha is our ignorance in regard to the excretory system. The sexual 

 organs, particularly those of the male portion of the body, exhibit an 

 important resemblance to the conditions in the Annelida, and if it is 

 l)ei-missible to refer the efferent sexual ducts to metamorphosed nephridia, 

 we should have to ascribe to Sagitta at least two trunk somites, and 

 accordingly explain the Chagtognatha as forms in which, perhaps in 

 connection with the manner of locomotion, a primitive segmentation of 

 the body has been retained in a degenerated form only. 



Embryologically considered, the Chsetognatha are distmguished from 

 the Annelida by the absence of a Trochophore-like embryonal or larval 

 stage, and, above all, by the characteristic folding off of the mesoderm. 

 In order to harmonize this kind of mesoderm formation with the de- 

 velopment of mesodermal bands in the Annelida, one would have to 

 assume that [in the Chaetognatha] the mesodermal elements increased 

 considerably by proliferation as early as in the blastula and gastrula 

 stages, so that in this way paired mesodermal bands arose, which at first 

 remained lying at the surface of the walls of the archenteron, retaining 

 an epithelial connection with the entoderm, and only later, by the forma- 

 tion of diverticula, became detached. By this assumption it is compre- 

 hensible how, even in closely related animals, two so apparently different 

 kinds of mesoderm formation might be realized. 



Literature. 



1. BuTscHLi, 0. Zur Entwicklungsgeschichte der Sagitta. Zeitschr. 

 wiss. Zool. Bd. xxiii. 1873. 



2. Gegenbaur, C. Ueber die Entwicklung der Sagitta. Ahh. Naturf. 

 Gesell. Halle, 1856. Translation : Quart. Jour. Micr. Sci. Vol. vii., 

 p. 47. 



3. Geassi, B. I Chfetognati. Fauna und Flora des GoJfes v. Keapel. 

 Monogr. v. Leipzig. 1883. 



