CHAETOGNATHA. 



587 



The body is elongated and divided into 

 the trunk, and the tail (Fig. 468). The 

 surface of the head, and has the form 

 of an elongated slit (Fig. 469). The 

 anus is on the ventral surface, at the 

 junction of the trunk and the tail. 



The head is slightly swollen, and is 

 partly covered by a prepuce-like fold of 

 the skin (Fig. 469), which arises from the 

 dorsal side of the head and forms a kind 

 of hood more developed laterally than 

 ventrally. It carries two laterally placed 

 groups of sickle - shaped setae and a 

 number of short spines. Moreover, there 

 are two eyes in close connection with 

 the cerebral ganglion. Close behind the 

 head on the dorsal surface there is an 

 annular modification of the ectoderm (Fig. 

 468 jR), the cells of which bear cilia; 

 this is supposed to be an olfactory organ. 

 The body possesses one or two pairs of 

 flat cutaneous fin -like folds the lateral 

 fins and the tail is surrounded by a 

 caudal fin. The fins consist of a fold 

 of the ectoderm containing a gelatinous 

 substance, on each surface of which lie 

 some chitinous rod-like rays beneath the 

 ectoderm. 



The skin consist.s of an epithelium, the 

 cells of which may be arranged in one 

 or in more than one layer, of a basement 

 membrane, and of a layer of longitudinally 

 disposed muscular fibres ; the latter are 

 disposed in the trunk and tail in four 

 bundles two dorsal and two ventral 

 (Fig. 470), and the fibres are cross-striped ; 

 these muscles, both in their arrangement 

 and appearance in transverse section, are 

 very like those of the Nematodes. In 

 the head the muscles are broken up into 

 bundles which work upon the jaws. 



three regions the head, 

 mouth is on the ventral 



R, 



FIG. 468. Spadella cephaloptera 

 (after O. Hertwig), dorsal view. 

 F lateral fin (in caudal region 

 only, in this genus) ; G cerebral 

 ganglion ; Te tentacles ; Ov 

 ovary ; Od oviduct ; T testis J 

 Vd vas deferens ; Sb vesicula 

 seminalis ; R olfactory organ. 



