THE SAGITTA. 133 
ing by a lateral duct on each side of the tail. The egg passes 
through a morula 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 _. Fis. 90.—Gastrula of Sa 
gitta.—After Kowalevsky. 
England. 
Cuass IIL—NEMATELMINTHES. 
Round-bodied worms, with a dense integument, not jointed ; with an ali- 
mentary canal (except in Hehinorhynchus); 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 Echinorhynchus and Gor- 
diacea no metamorphosis, the young hatching in the form of the 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 «2 mouth and 
intestine ; but no metamorphosis. Suborder 1. True WNe- 
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 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- 
scape. 
