VI 



PHYLUM NEMATHELMINTHES 



311 



a layer of muscles (w.), the fibres of which are striated and disposed longi- 

 tudinally in four bands two dorso-lateral and two ventro-lateral an 

 arrangement which recalls that of the corresponding layer in Nematoda. 



Enteric Canal. The mouth (Fig. 260, m.) ,-,,. 



is a longitudinal slit -like aperture on the ventral 

 surface of the head ; on either side of it are 

 several sickle-shaped chitinoid hooks (Fig. 262, 

 gh.) which are moved by muscles in a horizontal 

 plane and serve as jaws. The anterior region 

 of the head also bears spines, and is strength- we- 



ened by chitinoid plates and partly covered by 

 a hood-like fold of the integument. 



The mouth leads by a muscular pharynx or 

 stomodaeum into a straight intestine (d.), which 

 extends through the trunk and opens by the 

 anus (a) at the junction of trunk and tail. 



Coeiome. At the junction of the head with 

 the trunk, and of the trunk with the tail, are 

 transverse partitions or septa, dividing the 

 ccelome into compartments. The trunk region 

 of that cavity is further subdivided by two 

 longitudinal partitions, the dorsal and ventral 

 mesenteries, which respectively connect the dorsal 

 and ventral surfaces of the intestine with the 

 body-wall : the tail-region of the coelome is 

 similarly divided into right and left chambers 

 by a longitudinal vertical partition (Fig. 261, A 

 and B). 



There is no trace of vascular system or of ovtt- 

 excretory canals. The nervous system, on 

 the other hand, is much better developed than 

 in either of the preceding classes, in accordance 

 with a free life and active movements. On the 

 dorsal side of the pharynx is a comparatively 

 large 6mm (Fig. 262, g), which sends off on 

 each side a long nerve-cord, the cesophageal 

 connective (sc.). The two connectives sweep 

 round the enteric canal and unite on the ventral 



surface, not far from the middle of the trunk, "^ilf^lli 2 /to 



in an elongated ventral ganglion (Fig. 260, bg.), 

 from which numerous nerves are given off. The 

 brain sends nerves to the eyes (Fig. 262, an.) 

 and to the olfactory organs (ro.), and is also 

 connected with two pairs of ganglia in the head, 



which lie deeply sunk in the mesoderm : all the 



rest of the nervous system retains its primitive 



connection with the ectoderm. 



Sensory Organs. On the surface of the FIG. 



body are numerous little papillae carrying stiff 



bristle-like processes, and probably serving as 



organs of touch. There are two eyes (Fig. 263), 



situated one on each side of the dorsal surface 



of the head: each is globular and contains 



three biconvex lenses (L), separated by pigment 



(p.) and surrounded by rod-like sensory cells 



(rz.). Behind the head is a ring-like structure, 



of the nature of an annular ridge of peculiarly modified and in part ciliated 



cells (Fig. 262, ro.) : to this an olfactory function has been assigned. 



260. Sagitta hexap- 

 tera, from the ventral aspect 

 a. anus ; bg. ventral ganglion 

 d. intestine ; fl. lateral fins 

 ho. testis ; m. mouth ; ov.ovary 

 ovd. oviduct; sc. cesophageal 

 connective 4 sb. vesicula semin- 

 alis ; s. fl. tail fin ; sh, tail- 

 cavity; si. spermiduct. (From 

 Lang's Comparative Anatomy, 

 after Hertwig.) 



