Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 BALANOGLOSSUS. 301 the ectoderm, and sinks into a depression of the water vascular vesicle. At the apical pole there is a thickening of the ectoderm resembling the apical plate of the larval Worms and containing two eye-spots. The development of the larva into the adult Balanoglossus was first traced by E. Metschnikoff and then by A. Agassiz. The band of cilia gradually disappears, the prse-oral part of the larva becomes the proboscis, while the oral portion gives rise to the collar. The
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 BALANOGLOSSUS. 301 the ectoderm, and sinks into a depression of the water vascular vesicle. At the apical pole there is a thickening of the ectoderm resembling the apical plate of the larval Worms and containing two eye-spots. The development of the larva into the adult Balanoglossus was first traced by E. Metschnikoff and then by A. Agassiz. The band of cilia gradually disappears, the prse-oral part of the larva becomes the proboscis, while the oral portion gives rise to the collar. The trunk is formed from the posterior elongated portion on which the posterior transverse ciliated band still persists. The anterior portion of the alimentary canal becomes pierced by paired branchial slits (figs. 243, 244). The body of the adult animal is worm-like and completely cili- ated ; it can be divided by the external features into a number of different regions. The anterior end of the body is indicated by a proboscis well marked oft' and projecting like a FIG. in the con- head' Thi* is fo]- version of Tornaria into Bala- lowed by a 1T1USCU- noglossns, with one pair of branohial slits (after E. Met- schnikoff), seen from the side. So, external branchial open- ing ; p, peritoneal sac ; Vc, there is a longer portion of the body, the branchial region, which may be divided into J a median distinctly ringed part (branchiae) and two lobed lateral portions usually filled with yellow glands. At the boundary, between the median portion and the two lateral lobes, there are on either side rows of openings which serve for the exit of the water from the branchial chamber. Then follows a third division of the body, the gastric region, upon the upper side of which there are four rows of yellow glands (generative glands}.
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