. Elementary text-book of zoology. Zoology. 359 of which two cells may already be recognised as primitive generative cells. As soon as these make their appearance in the entoderm, the latter 1 tecomes folded in such a way that the archenteron is divided into a median and two lateral cavities. The layer of cells lining the lateral cavities becomes the mesoderm, and the contained cavities the two lateral compartments of the body cavity, while that of the middle cavity gives rise to the wall of the mesenteron or alimentary canal. The permanent mouth is formed at the end opposite t
. Elementary text-book of zoology. Zoology. 359 of which two cells may already be recognised as primitive generative cells. As soon as these make their appearance in the entoderm, the latter 1 tecomes folded in such a way that the archenteron is divided into a median and two lateral cavities. The layer of cells lining the lateral cavities becomes the mesoderm, and the contained cavities the two lateral compartments of the body cavity, while that of the middle cavity gives rise to the wall of the mesenteron or alimentary canal. The permanent mouth is formed at the end opposite to that at which the blastopore, which is now closed, was situated. There is but one genus, Sagitta Slab., of which several species, , tiagitta bipunctata Krohn, tf. germanica Lkt. Pag. from the Euro- pean seas have been more accurately described. Order 2.—ACANTHOCEPHALA.* Elongated round worms with protrusible proboscis furnished with hooks; without mouth and alimentary canal. The saccular, often transversely wrinkled body begins with a proboscis, which is furnished with recurved hooks and can be retracted into a tube projecting into the body cavity (sheath of the proboscis) (fig. 289, R and Rs]. The posterior end of this sheath is fas- tened to the body wall by a ligament, and by retractor muscles. The nervous system (fig. 289, G) is placed at the base of the proboscis, and consists of a simple ganglion formed of large cells. Nerves are given off from the ganglion anteriorly to the proboscis, and through the lateral retractors (retinacula) to the body wall g_ (fig. 289, R). The latter supply partly the muscular system of the body, and partly the Rj genital apparatus, in which there are, princi- pally in the male animal, special nerve centres FIG. 289—Anterior part, • .• f ,. , of an Echinorhtinchus. consisting of ganglionic enlargements. R> Proboscis; X*, Sense organs are entirely wanting, as also are sheath of proboscis; ,. , i G, gang-lion; Le, lem mouth, alim
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
