Till: DKVKLol'MKXT OF SAGITTA. 643 



with a largo cleavage cavity, or blastocoele. One face of the 

 vesicle thus constituted now becomes iuvaginatcd, \\itli thr 

 effect of gradii;i lly obliterating the blastoccele, and converting 

 tin- spherical single-walled sac into a hemispherical, double- 

 walled, cup-shaped gastrula. The cavity of the cup is the 

 future digestive cavity ; the layer of invaginated blasto- 

 dermic cells which lines this cavity is the hypoblast, which 

 will become the endoderrn ; and the outer layer of cells is 

 the epiblast, and will become the ectoderm. In this condi- 

 tion the embryo resembles that of the Leech in its early 

 state. The embryo elongates, and the aperture of in vagi- 

 nation, or blastopore, eventually ceases to be discernible. 

 Whether it becomes the anus, or whether the anal aperture 

 is formed anew, is not certain. The nervous ganglia result 

 from the modification of cells of the ectoderm. The anterior 

 end of the primitive alimentary cavity, or archenteron, is at 

 first closed. It soon sends out an enlargement on each side, 

 so that the archenteron is divided into a central and two lat- 

 eral divisions. The central division opens externally and an- 

 teriorly by the development of the oral aperture ; and, as the 

 body elongates, it becomes the tubular intestine. The lat- 

 eral diverticula at first communicate with it, but they are 

 eventually shut off, and constitute the right and left perivis- 

 ceral caviiies, their walls becoming converted into the cellu- 

 lar and muscular lining of those cavities. It results, from the 

 mode of development of the perivisceral cavity of Sagiita, 

 that this cavity, like the perivisceral cavity of the Brachio- 

 pods, and the " peritoneal " cavity of the Echinoderms, is an 

 enteroccele, comparable to that of the Hydrozoa and Actino- 

 zoa ; but which, instead of remaining in communication with 

 the alimentary cavitv, is shut off from it, its wall becoming 

 the mesoderm, and its cavity the perivisceral cavity. 1 



Nothing of this kind is known to occur in the Turbellaria, 

 Annelida, Nematoidea, or Rotifera ; but when a perivisceral 

 cavity exists in these animals, it appears always to result from 



primitive enterocoele into two sacs one for' the head and another for the body. 

 It appears ]>n>l>;il>le that the latter becomes subdivided by a transverse parti- 

 tion between the ovary and testis. Biitschli suggests that the segmentation 

 of the mesoblast, which forms the walls of the enteroea'le, is a point of approxi- 

 mation between Sagitta and the Annelids. 



* " Zur EntwickeUmgsgeschichte der Sagitta." (ZeUscMft fur tris*. Zootogk, 



1873.) 



