VERMES 



125 



is usually passed in some insect or similar small 

 aquatic animal, and enters its second host, where it 

 becomes sexually mature when the latter swallows 

 the first host. Some of the Acanthocephala occur 

 in birds, and the largest is found in the pig ( Fig. 

 115), where it may attain the size of the Nematode, 

 Ascaris, found in the same animal. 



-6 



SUBTYPE IV. CfTETOGNATHA 



The Chaetognatha(Gr. x aiT V> bristle, and yvdOos, 

 jaw) have long troubled the systematic zoologist, 

 for they clearly belong to none of the recognized 

 subtypes of Vermes, and for this reason it seems 

 best, though the group is a small one, to assign it 

 a subtype of its own. They are small, transparent, 

 marine animals, with a rounded body from one to 

 five centimeters long, very slender, and best known 

 through the genus Sagitta, the " arrow-worm," 

 which darts about near the surface of the water 

 (Fig. 1 16). They possess paired, finlike outgrowths 

 on the sides of the body, projecting horizontally, 

 which vary in number in the different species. 

 The mouth is on the ventral side, near the anterior 

 end of the body, and is provided on each side with 

 a fold, which bears several stiff bristles, directed 

 forward, and these give the name Chaetognatha. 

 The alimentary canal is straight, terminating in an 

 anus on the ventral side, but not at the extreme pos- 

 terior end of the body. A body-cavity is present, 

 divided by a dorsoventral membrane into a right 

 and left half, and further separated by transverse 

 membranes into three compartments or segments, 

 — head, trunk, and tail portions, the anus being at 

 the extreme posterior end of the trunk segment. 

 There are no circulatory or excretory organs, hut 

 the nervous system is well developed ; it consists oi 

 a brain in the dorsal part of the head, from which 

 fibers pass to the two eyes and other sensory or- 

 gans, and two long connectives extend around the 



-efc 







—j 



-k, 



FIG. 116. Sag it id 

 hexaptera, enlarged 

 three am] a half di- 

 ameters; ventral 

 aspect. .;. mouth ; 

 #, intestine; c.anus; 

 ,/, ventral ganglion ; 

 r. bristles;/! spines; 

 g t ovary; h, ovi- 

 duct ; /. vas 

 ferens , \ testis; k, 

 vesicula seminalis. 

 ( From Shipley and 

 MacBride's 2 



