THE SAG ITT A. 175 



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

 through a morula and gastrula stage (Fig. 120). The i^rim- 

 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 elcgans Ver- 

 rill is about 16 millimetres in lenarth, 

 and is common in the waters of ISTew Fig- lao.-Gastruia of sa^ 



Eno-land. gitta-After Kowalevsky. 



Class II.— NEMATELMINTHES. 



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

 mentary canal {except in EcMnorhynchus); no water -vascular w respira- 

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

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

 sometimes hooTced or spinulated ; and except in EchinorhyncKus and Qor- 

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

 Mostly parasitic, and usually bisexual. 



Order 1. Acanthoeephali. — 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, Tricliocephalus, 

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

 Oordiacea (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 tlie micro- 

 scope. 



