THE SAGITTA. 175 



ing by a lateral duct on each side of the tail. The egg passes 

 through a morula and gastrula stage (Fig. 130). The prim- 

 itive opening (a) afterwards closes 



and a new opening is made at the op- y-cOfffRP~X 



posite pole, which is the permanent /$^^:ra^r^^<50^ 



mouth. The embryo is oval at first, f^^S^^^^oAW^ 



but soon elongates, and the form of the ez/^S H^Q 



adult is attained before the Sagitta ^^Si, i /S^R 



leaves the egg. Sagitta elegans Ver- ^^^^J— ^"v''^ 



rill is about 16 millimetres in length, x:n-^ ^ ^<j-^ 



and is common in the waters of New I'ig- lao.-Gastruia of sa- 



T-, 1 T gitta. — After Kowalevsky. 



England. 



Class II.— KEMATELMINTHES. 



Bmind-bodied worms, with a dense integument, not jointed / witJi an ali- 

 mentcvry canal (except in Echinorhynchui); no water-vascular or reapira- 

 tory system ; tlie nervous system usually reduced to a brain and two ner- 

 vous threads passing along ilie body ; with excretory organs. The head 

 sometimes hooked or spinulated ; and except in Echinorhynchus and Qor- 

 dicboea no metamorphosis, the young hatching in the form, of the adult. 

 Mostly paradtie, and usually bisexual. 



Order 1. Aeanthocephali. — Cylindrical, with a beak armed with hooks, 

 without mouth or digestive tract. (Echinorhynchus.) 



Order 2. Nematodes. — Long, slender, cylindrical, with a mouth and 

 intestine ; but no metamorphosis. Suborder 1. True Ne- 

 matodes (Ascaris, Oxyuris, Eustrongylus, Trichocephalus, 

 Trichina, Pilaria, Anguillula, Echinoderes). Suborder 3. 

 Oordiaeea (Mermis, Gordius). 



Order 3. Chaetognathi. — Having a well-marked head, with lateral and 

 caudal fln-like expansions of the skin ; hermaphrodite. 

 (Sagitta.) 



Laboratory Work. — These worms are to be mainly sought for in 

 the alimentary tract of fishes and mammals, while Sagitta may be 

 caught with the tow-net. They may be studied with good success be- 

 sides the ordinary mode of dissection, by cross-sections for the micro- 

 scope. 



