THE SAGITTA. 175 



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

 through a mornla and gastrula stage (Fig. 120). The prim- 

 itive opening (a) afterwards closes 

 and a new opening is made at the op- 

 posite pole, which is the permanent e 

 mouth. The embryo is oval at first, 

 but soon elongates, and the form of the en \ 

 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 r >s- 120. Gastmia of sa- 



gitta. After Kowalevsky. 



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; the 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 EchinorJiynchus and Gor- 

 diacea no metamorphosis, tlie young Jiatching in the farm of tJie adult. 

 Mostly parasitic, and usually bisexual. 



Order 1. Acanthocephali. 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, Anguillula, 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 track 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. 



