THE SAG ITT A. 175 



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

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

 itive opening (a) afterwards closes 

 and a new opening is made at the op- X5?ff[°^2x 



posite pole, which is the permanent /^^^^^^^V%\ 

 mouth. The embryo is oval af first, i^^^^"^^^^j 

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



adult is attained before the Sagitta ^^^. i /S^O 

 leaves the Qgg. Sagitta elegans Ver- ^^^^'4-.^i%X 

 rill is about 16 millimetres in length, ^CCP' ^ 



and is common in the waters of Kew rig- iso.-Gastmia of sa- 



-_, T T gitta. — After Kowalevsky. 



JSngland. 



Class II.— NEMATELMINTIIES. 



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

 mentary canal {except in Eahinorhynehus); no water-Dascular or respirOr. 

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

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

 sometimes hooked or spinulated; and except in Bchinorhynchus and Oor. 

 diacea no melamorpliom, the young luitching in the form of tlie adult, 

 MosUy parasitic, and usually bisexual. 



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

 without mouth or digestive tract. (Echinorhynchus.) 



Order 3. Nematodes. — Long, slender, cylindrical, with a mouth and 

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

 matodes (Ascaris, Oxyuris, Eustrongylus, Trichoceplialus, 

 Trichina, Pilaria, Anguillula, Echinoderes). Suborder 2. 

 Oordiacea (Mermis, Gordius). 



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

 caudal fln-like expansions of the skin ; hermaphrodite. 

 (Sagitta.) 



Laboratory Worh. — 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. 



