. Zoology : for students and general readers . Zoology. 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- of sa^ Eno-land. gitta-After Class
. Zoology : for students and general readers . Zoology. 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- of sa^ Eno-land. gitta-After 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- Please note that these images are extracted from scanned page images that may have been digitally enhanced for rea
Size: 1696px × 1474px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1879
