CILETOGIfATIIA. 



359 



of which two cells may already be recognised as primitive generative 

 cells. As soon as these make their appearance in the entoderni, the 

 latter becomes folded in such a way that the archenteron is divided into 

 a median and two lateral cavities. The layer of cells lining the lateral 

 cavities becomes the mesoderm, and the contained cavities the two 

 lateral compartments of the body cavity, while that of the middle 

 cavity gives rise to the wall of the mesenteron or alimentary canal. 

 The permanent mouth is formed at the end opposite to that at 

 which the blastopore, which is now closed, was situated. 



There is but one genus, Sagitta Slab., of which several species, e.tj., 

 Sagitta bipunctata Krohn, 8. yermanica Lkt. Pag. from the Euro- 

 pean seas have been more accurately described. 



Order 2. ACANTHOCEPIIALA.* 



Elongated round ivonns with protrusible proboscis furnisJied with 

 hooks; without mouth and alimentary canal. 



The saccular, often transversely wrinkled body begins with 

 a proboscis, which is furnished with recurved hooks and can be 

 retracted into a tube projecting into the body 

 cavity (sheath of the proboscis) (fig. 289, ^ and 

 jRs}. The posterior end of this sheath is fas- 

 tened to the body wall by a ligament, and by 

 retractor muscles. The nervous system (fig. 289, 

 G) is placed at the base of the proboscis, and 

 consists of a simple ganglion formed of large 

 cells. Nerves are given off from the ganglion 

 anteriorly to the proboscis, and through the 

 lateral retractors (retinacula) to the body wall 

 (fig. 289, R}. The latter supply partly the 

 muscular system of the body, and partly the RJ 

 genital apparatus, in which there are, princi- 

 pally in the male animal, special nerve centres FIG. 289 Anterior part, 



. . . ,. ..., of an Eclnnorhiinchus. 



consisting of ganghonic enlargements. _ R> Prob08Ci8 . Sti 



Sense organs are entirely wanting, as also are sheath of proboscis ; 



.. , <?, ganpliou ; Le, lem- 



mouth, alimentary canal, and anus. nisci . R> re ti na cuia. 



The nutritive juices are taken in through the 

 whole outer surface of the body. In the soft granular subcuticular 



* Besides 

 Menschen 



ides Dujardin, Diesing, 1. c., compare : K. Leuckart, " Parasitcn des 

 en," Tom II., 1876. Greeff, " Untersuchungen Ubcr Echinorhyncbus 

 miliaris," Arch, fur Naturgeseh, 1864. A Schneider, Ueber den ] 

 Acanthocephalen," Mutter'* Arckiv., 1868. Also the SUzungslertchte der 

 Oberliessischen Gesellsc liaft filr Natnr- und Ileilkundc, 1871. 



