. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. BALAArOGLOSSUS. 247 Far from assuming as a self-evident fact that the extreme metamerism of the AnneHds and Arthropods is genetically identical with that of the Vertebrates, we have every reason to suppose that it has been elaborated entirely independently in the two cases, and that the apparent simi- larity is due, as already intimated, to a parallel evolution. Body-cavities; Proboscis-pore ; Collar-pores. Corresponding to the three regions into which the body of Balanoglossus is divided, â namely, probos-
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. BALAArOGLOSSUS. 247 Far from assuming as a self-evident fact that the extreme metamerism of the AnneHds and Arthropods is genetically identical with that of the Vertebrates, we have every reason to suppose that it has been elaborated entirely independently in the two cases, and that the apparent simi- larity is due, as already intimated, to a parallel evolution. Body-cavities; Proboscis-pore ; Collar-pores. Corresponding to the three regions into which the body of Balanoglossus is divided, â namely, probos- cis, collar, and trunk, â the body-cavity is divided up into three systems of cavities. These are (a) the anterior body-cavity or cavity of the proboscis, (;S) a pair of collar- cavities, and (7) a pair of body-cavities which form the unsegmcnted ccelom of the trunk (Figs. 114, 115). These cavities arise essen- tially as pouches from the archenteron (Fig. 117), al- though their actual develop- . ^-re -1 11- FiK. 116. â Diarram of the or^anisa- mcnt differs considerably m âââ ^^ Baianogiossus, from the dcsai different species (Morgan), side. (From a drawing kindly lent by Professor T. H. MORGAN.) The proboscis-cavity is Collar-pores. go. Gonads. 11- â .â Gill-slits: the dark linos converginEr be- placed in communication ^^^j j^^'^^^,^ â^^ superficial portions of with the exterior by an open- "i« gill-sHts; bcIow the surface arc seen the hee ends of the tongue-bars. //. ing through the posterior l>roboscis-porc. Other letters as Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Willey, Arthur, 1867-. New York, London, Macmillan
Size: 940px × 2658px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1894
