. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. 128 DEVELOPMENT OF AMP/I/OX US. '"€ ! I! (1 I.' • li. f t i' 1 II; i! Ill- 'H ra «? CJ.^. ;f and constitutes the cavity of the snout lying below the notochord (Fig. 63 C). Shortly after the breaking through of the mouth the left sac acquires an opening to the exterior on the left side of the body (Fig. 64). The right sac becomes thQ fraornl body-cavity or coelom of the ' head," while the left sac is known as the pnvoral pit. It is nece
. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. 128 DEVELOPMENT OF AMP/I/OX US. '"€ ! I! (1 I.' • li. f t i' 1 II; i! Ill- 'H ra «? CJ.^. ;f and constitutes the cavity of the snout lying below the notochord (Fig. 63 C). Shortly after the breaking through of the mouth the left sac acquires an opening to the exterior on the left side of the body (Fig. 64). The right sac becomes thQ fraornl body-cavity or coelom of the ' head," while the left sac is known as the pnvoral pit. It is necessary to emphasise the fact that these two structures which are so different in their fully formed con- dition are at first perfectly similar and symmetrical and form a pair of "head-cavi- ; Ultimately, as we have seen, only one of them actually persists as a head- Fig. 71.— Anterior portion of em- cavity; namely, the right one. brvo, witii thirteen primitive somites, nni . • • r from the ventral side in optical section! ^hc entire COnVCrSlOn of (After hatschkk.) the left sac into the prreoral and Right and left head- . cavities, Rudiment of club-shaped pit IS probably to be regarded ^''^"'" as a secondary or cenoge- netic phenomenon, but the acquirement of an opening to the exterior is probably not secondary, since a smiilar opening (the proboscis-pore) occurs in Balanoglossus. Tn addition to the above-described peculiarities which sufficiently distinguish the head-cavities from the myocoe- lomic pouches, must be mentioned the fact that at no point of their epithelial walls are muscles developed. It is probable that the prceoral head-cavities of Amphi- oxus are homologous with the prcemandibular cavities of the higher Vertebrates, from the walls of which the greater number of the eye-muscles are developed.* This view is * This is also the opinion of Kupffcr. Singularly enough van Wijhe has advanced the view that only
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectfishes, bookyear1894
