A treatise on zoology . very far back (Fig. 31).This migration of the branchial pouches behind the first three, whichdisappear in situ, occurs somewhat late in development (Dean[106]). Consequently the gill-openings are pierced between thedorsal and ventral somatic muscles, irrespective of their metamericorder, when the gills reach their final position (the branchial nerves,of course, follow the gills). There is always on the left side asimple tube leading from the pharynx to the exterior, and open-ing in common with the last gill-pouch—it is the oesophageo-cutaneous duct, probably a modified
A treatise on zoology . very far back (Fig. 31).This migration of the branchial pouches behind the first three, whichdisappear in situ, occurs somewhat late in development (Dean[106]). Consequently the gill-openings are pierced between thedorsal and ventral somatic muscles, irrespective of their metamericorder, when the gills reach their final position (the branchial nerves,of course, follow the gills). There is always on the left side asimple tube leading from the pharynx to the exterior, and open-ing in common with the last gill-pouch—it is the oesophageo-cutaneous duct, probably a modified gill-slit (Figs. 23, 27). Thegills, in Bdellostoma, open independently to the outside, and theremay be as many as fourteen pairs. But in Mtjxine, where there MYXINOIDEA 47 are, as a rule, only six pairs, the elongated external ducts passIjackwards, and open by a common pore on each side. The genusParamyxine (Fig. 31) shows an admirably intermediate stage inthe evolution of this secondary arrangement (Dean [110]). \S^-mt. VTfh- rns:
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
