LA.Ml'ltlCYS. 



117;-! 



irreat inorplKiloiiic'il signitirMiicc. in tlic structure of 

 the skeleton of tlic Laniin-cys the difference between 

 the internal (verteliral) and tiic external (extravertebral) 

 parts is far inure |ir(iniinent than in the tishes we have 

 hitherto considered, the latter parts being here endowed 

 with a more prei)onderating iinjiortance. The purely 

 vertebral parts appear in the LaniprcAs merely as the 

 framework of tiie cranium and its nearest appendages 

 and as very imperfectly developed neural arches (fig. 

 .143, arc) on the sides of the myelon and the unaltered 

 notochord (etch)- I" ^lie vertical fins too cartilaginous 

 rods are developed which arc applied to the neural 

 arches in such a manner that it has been proposed to 

 interpret them as representing spinous processes both 

 superior (neural) and inferior (hajmal). The skull itself 

 is an almost straight continuation of the fibrous me- 

 dullispinal sheath, which has ])een dilated, and which 



grows, on each side of the brain, t(j forni a moi'e or 

 less complete i-oof for the cerebral cavity {tetjmen cranii, 

 t). Laterally and downwards the floor expands in three 

 places on each side into flat ]>rocesses. The fonnnost 

 ])air of these })rocesses projects from the ethmoidal pai-t, 

 the posterior two pairs issue from the spheiKjidal jnirt. 

 'Hie former is I'vidently homologous with a palatine arch 

 ((xil), the latter answei' to the upper parts in the sus- 

 pensory apparatus of the mandibular and hyoid arches. 

 But no mandibular arch proper is developed in these 

 fishes. The two posterior processes on each side are 

 confluent at the base; but the antei'ioi' of them (jiff/) 

 grows outwards and forwards, in a loop below the eye, 

 Tuitil it coalesces with the top of the foremost (palatine) 

 process (pal). In this manner a flat ring is formed, 

 surrounding an aperture {fenestra suhorbliaJ'is ov fissura 

 orhitaUs inferior, fo), and composing the bottom of the 



la ctr Ip net > < 



Fig. 343. The eartiliigincus skeleton of the anterior part of a Lampern (PHromijson fliwiatilh), ^, ., naf. size. 

 mm, annular cartilage of the oral disk; rtrf, rudimentary neural arches; a», cartilaginous capsule of the auditory apparatus; br, branchial basket; 

 i-eh, eeratohyal cartilage; cir, posterior cartilage of the roof of the mouth {conma trabecxdarum); eph, epihyal cartilage; et, ethmoidal pait 

 of the cranial floor; etch, outer sheath of the chorda dorsalis; fo suborbital fenestra; la, anterior lateral cartilage of the roof of the raouth ; 

 lat, lateral labial cartilage; //), posterior lateral cartilage of the roof of the mouth; meil, median cartihige of the roof of the raouth; n, ol- 

 factory capsule, with the tubular orifice of the nasal duct; //a/, palatine part of the roof of the mouth; phy, posterior hyoid cartilage; 

 pre, pericardial cartilage; pUj, pterygoid part of the roof of the mouth; st, sublingual cartilage; sph, sphenoidal part of the cranial Hoor; 

 spl, symplectic (^? hyomandibular) cartilage; t, lower part of the cranial roof (legmen cranii). 



is invested in the developed Lamprey with a more or 

 less comjilete cartilaginous capsule (0 originating from 

 the skeletogenous laj'er round the anterior end ot tiie 

 notochord. The part first formed, the cranial floor, 

 answering to a sphenoidal region (spli) applies itself to 

 and coalesces with the already present auditory capsules 

 (au). In a forward direction are developed the so-called 

 cranial trabecukv, around the cerebral appendage (hypo- 

 physis, posterior nasal duct) and the under-brain 

 (infundibuluni); and after their union in front into an 

 ethmoidal part (i-t) they advance to the so-called corniia 

 frahecularuni {ifr), which coalesce into a disk in front 

 of the nasal capsule («)• Upwards the cranial floor 



cranial orbit. To the top of the hindmost process is 

 attached the hyoid arch proper. The upper part of 

 the latter is an unbroken downward continuation of the 

 said process, and represents an epihyal bone [ejih); the 

 lower (ventral) jiart, which is suspended at right angles to 

 the lower tip of the upper part, answers to a eeratohyal 

 bone (ceh). A certain similarity to this structure of 

 the cranial roof of the palate and pharynx we have 

 indeed seen in the Chimtvras (fig. 294, p. 1U65), where 

 a coalescence, suggestive of a dev<!lopment (jresumably 

 very primitive, has taken place between the pterygo- 

 palato-quadrate parts and the cranium. But the closest 

 resemblance to tliis structure in the Lampreys meets us 



