BRAIX. GILLS. 



163 



regarded as enamel. These scales in the young state and some- 

 times throughout life bear spines which project through the 

 epidermis and are formed of dentine capped by enamel (Poly- 

 pterus, Lepidosteus}. (11) The fins frequently, not always, possess 

 a single or double row of spine-like scales, called fulcra, on their 

 anterior edge (absent in Polypterus, Polyodon and Amia). The 

 dermotrichia are soft and segmented, and the pelvic fins are 

 abdominal. (12) They lay relatively small eggs which undergo 

 complete cleavage, and the young are hatched out as larvae 

 which differ in many respects from the adult and possess a prone- 

 phros as in Teleosteans. In Lepidosteus the medullary canal 

 arises as a solid keel-like projection of the ectoderm which 

 sub sequently 

 becomes hol- 

 low. 



The brain 

 is on the 

 whole Teleos- 

 tean - like . 

 There is a 

 thin pallium 

 (Fig. 91) and 

 a val vul a 

 cerebelli and 

 the cerebrum 

 is undivided ; 

 though it 



may be grooved externally so as to suggest a division into two 

 lobes. 



The branchial apparatus presents remarkable variation. In 

 Spatularia (Planirostra edentula) there is no hyoid gill, but a 

 pseudobranch in the spiracle as in Acipenser. In Polypterus 

 there is no hyoid gill, nor spiracular pseudobranch. The spiracle 

 is present in Acipenser, Polyodon and Polypterus, but absent in 

 Scaphirhynchus, Lepidosteus and Amia. The following table 

 summarizes the matter : 



Hyoid gill, pseudobranch and spiracle . Acipenser. 



Hyoid gill, pseudobranch* but no spiracle Lepidosteus. 



* It is doubtful if the structure identified by Muller as pseudobranch in 

 Lepidosteus is really such. 



FIG. 91. Median section through the brain of Acipenser ruthenus 

 (from Gegenbaur, after Goronowitsch). Cb cerebellum ; ch 

 optic chiasira ; cp pineal body ; hy pituitary body ; pi pal- 

 lium with choroid plexus projecting in between the" third ven- 

 tricle z and the ventricle of the cerebrum V ; pi' roof of fourth 

 ventricle ; Sv saccus vasculosus. 



