THE 8AGITTA. 133 



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

 through a mornla and gastrula stage (Fig. 90). The prim- 

 itive opening (a) afterwards closes 

 and a new opening is made at the op- 

 posite pole, which is the permanent 

 mouth. The embryo is oval at first, 

 but soon elongates, and the form of the 

 adult is attained before the Sagitta 

 leaves the egg. Sagitta elegans Ver- 

 rill is about 16 millimetres in length, 

 and is common in the waters of New lt 

 England. 



CLASS II. NEMATELMINTHES. 



Round-bodied worms, with a dense integument, not jointed ; with an ali- 

 mentary canal (except in Echinorhynchus); no water-vascular or respira- 

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

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

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

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

 Mostly parasitic, and usually bisexual. 



Order 1. AcanthocepJiali. 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, Filaria, Angurllula, Echinoderes). Suborder 2. 

 Gordiacea, (Mermis, Gordius). 



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

 caudal fin-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 )>e 

 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. 



