. The causes and course of organic evolution; . transition from nemerteansto primitive cyclostome vertebrates, and of a remainder beingutilized in other ways. We may best ascertain if such is likelyafter examination of the proboscis when at rest, and whatmight result if, by gradual filling up of the sheath as we seeproceeding in Cerebratulus, it thus becomes converted sec-ondarily into a dorsal sensitive and strengthening oigan, thuscausing the entire proboscis to be extruded and again unitedwdth the oral cavity. Plate Fig. 15, f is a diagram of the proboscis extruded infront of the now solid
. The causes and course of organic evolution; . transition from nemerteansto primitive cyclostome vertebrates, and of a remainder beingutilized in other ways. We may best ascertain if such is likelyafter examination of the proboscis when at rest, and whatmight result if, by gradual filling up of the sheath as we seeproceeding in Cerebratulus, it thus becomes converted sec-ondarily into a dorsal sensitive and strengthening oigan, thuscausing the entire proboscis to be extruded and again unitedwdth the oral cavity. Plate Fig. 15, f is a diagram of the proboscis extruded infront of the now solid notochord, and separating into the pos-terior glandular portion that is pushing upward from the roof ofthe mouth, and underneath the infundibulum. This trans-formation seems a necessary step in evolution from the nemer-teans to a group somewhat distantly related to the cyclostomesand which in the figure we have termed a paracyclostome anterior and originally inverted part of the proboscis - Jz ~ > Z ^ S Si „ c S 3 i- > £ - c?. Evolution of Animals 425 becomes now the lining of the oral cavity, and as in some nemer-teans retains the capacity to develop strong muscles and tactilepapillae, also glands for excretion of the juices into the oralcavity. The mid region of the proboscis, the so-called tonguein MyxinCy retains the capacity for development of pharyngealteeth, either a median homy tooth as in Myxine, or a seriesspread over the entire area as in some nemerteans and in Petro-myzon, or vomerine and other accessory buccal teeth as inBatrachia. Further the sunken glands of this posterior buccal region,where it joins on to the oesophagus in nemerteans, may wellrepresent the rudiments of the thyroid and of the thymusglands of vertebrates. A critical and detailed study of thesein relation to vertebrate cephalic glands is highly desirable. That the notochord of vertebrates has somehow originatedas a structure that primarily had a different function has oftenbeen e
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