A treatise on zoology . i S•-^ ^ ° >- ?;~S ^ o ^ ^ c^ 5 ^ o 8 « c .. ^^„ a ^ P CO S ff M S 5 a S ° e =2 •== o ° r,,c 2 tu; -c \ C3 to O * ^ s- » J4 s « S 2 C3 « .2 ^ o t, -g ^ *^ -?^ • C 5 c3 C3 CS , ?^ B, -to JP bD -^ > S « !* S -5 o! ^;.5 -? o o nor horny armature, nor yet any rasping tongue. The buccalcavity is separated by a velum from the pharynx, and this is limitedin front by an encircling ciliated groove (like that of Amphioxus),which is at the level formerly occupied by the transitory first gill- PE TROM YZONTIA 53 slit. The groove is carried back along the floor of the pharynx
A treatise on zoology . i S•-^ ^ ° >- ?;~S ^ o ^ ^ c^ 5 ^ o 8 « c .. ^^„ a ^ P CO S ff M S 5 a S ° e =2 •== o ° r,,c 2 tu; -c \ C3 to O * ^ s- » J4 s « S 2 C3 « .2 ^ o t, -g ^ *^ -?^ • C 5 c3 C3 CS , ?^ B, -to JP bD -^ > S « !* S -5 o! ^;.5 -? o o nor horny armature, nor yet any rasping tongue. The buccalcavity is separated by a velum from the pharynx, and this is limitedin front by an encircling ciliated groove (like that of Amphioxus),which is at the level formerly occupied by the transitory first gill- PE TROM YZONTIA 53 slit. The groove is carried back along the floor of the pharynx intothe opening of the thyroid gland. This gland develops as a mid-ventral outgrowth of the pharynx, acquires a lumen of considerablesize, and along its folded walls become differentiated four rows ofmucous cells (Fig. 36). In fact, the whole structure bears astriking and unmistakal)le resemblance to the endostyle of the. hh af th Fig. 35 Transverse section of the .iiill-region of an Ammocoete larva, somewliat diagrammatic. (Partlyafter Alcoclc.) , anterior cardinal vein ; of, afferent artery ; , branchial basket; , dorsalaorta ; , dorsal blood-sinus ; e/, efferent artery ; g, gill-lamella ; , gill-opening ; , lateral-line nerve; n, nerve-cord; nt, uotochord ; th, thyroid gland; , ventral aorta; , vagusnerve ; , ventral blood-sinus. Tunicata and Cephalochorda, with which it is no doubt homologous(W. Miiller [307((]). It may also be mentioned that the liver iscomposed of a mass of branching tubules, much more distinct thanin the compact liver of higher vertebrates ; in the adult it undergoesfatty degeneration, and the gall-bladder is lost. The paired eyeslie hidden deep beneath the skin. The pericardium opens widelyinto the abdominal coelom (Fig. 37, C); on its dorsal wall lies thepronephros. The median fin is continuous. 54 PE TROM YZONTIA Both marine and freshwater Lamprey
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
