A treatise on zoology . ralvalve in the intestine, the subdivision of the dorsal fin, etc., havebeen mentioned above. It is in the Ammocoete larva of Petromijzon that some of thestrongest evidence of the primitive character of the Cyclostomes isfound to occur (Balfour [29], Scott [393], Dohrn [11G], von Kupffer[275], Koltzoff [273], Schneider [389], etc.). The eggs are thin-shelled, small, with comparatively little yolk,and undergo holoblastic cleavage. The blastopore becomes theanus. The first three mesoblastic somites develop as outgrowthsof the archenteric wall, and contain a coelomic cavit
A treatise on zoology . ralvalve in the intestine, the subdivision of the dorsal fin, etc., havebeen mentioned above. It is in the Ammocoete larva of Petromijzon that some of thestrongest evidence of the primitive character of the Cyclostomes isfound to occur (Balfour [29], Scott [393], Dohrn [11G], von Kupffer[275], Koltzoff [273], Schneider [389], etc.). The eggs are thin-shelled, small, with comparatively little yolk,and undergo holoblastic cleavage. The blastopore becomes theanus. The first three mesoblastic somites develop as outgrowthsof the archenteric wall, and contain a coelomic cavity which for atime is in communication with the archenteron as in embryo develops into a larva differing very considerably instructure from the adult, and undergoes a pronounced metamor-phosis to reach the perfect state. In this Ammocoete many 52 PE TROM YZONTIA remarkable features bridge over the gulf between the Craniataand the Cephalochorda. The mouth is bounded by lips (Fig. 37); there is neither sucker. 3 c! o SO ^ -^ 2 ^ 5;i ^ S C! a. SJ SS OT ^ t, S 5 5 _=--& IS cS .? ., t- S S g a3?o — ; * 5 ii 5. > - 5^i3^ OJ o ^ -^ -e -P to ^ .S t. is c =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 b
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