229 



Hippocampus (Sea-horse). Pelagic. The 

 male has an external pouch in which 

 the eggs are hatched. 

 Fierasfer. Lives inside medusae and 



sea-cucumbers (commensalism). 

 Anabas (Climbing Perch). Oriental 

 Physoclisti Region. Has superior pharyngeal 



(Duct of air- bones hollowed out for holding 



bladder water, an accessory means of re- 



closed), spiration used by this fish when it 



moves about on land. 

 Gadus (Haddock, Cod). 

 Pleuronectes (Plaice). Body flattened 

 from side to side. Without an air- 

 bladder. 

 Zoarces (Viviparous Blenny). 



Sub-class DIPNOI (Double breathers). With gills and a lung (single 

 or double), which opens into the pharynx mid-ventrally. Body- 

 covered with overlapping bony scales. Paired fins of archi- 

 pterygium type (seepage 194). Tail protocercal, and the vertical 

 caudal fin extending more or less along the body dorsally and 

 venbrally. External nares within the upper lip ; the nasal sac 

 also opens into the buccal cavity anteriorly by internal nares. 

 The internal gills covered, each side, by an operculum. Four 

 to six branchial arches. Notochord persistent, with an un- 

 segmented sheath, and with partly ossified neural arches, ribs 

 and haemal arches. The protostylic skull is a chondrocranium 

 with a few supplementary investment or membrane bones, 

 and without maxilla and premaxilla. Teeth, composite, are 

 three pairs (mandibular, palatal and vomerine) of peculiar 

 dental plates with ridges or cusps. A spiral valve in intestine. 

 Gut, and renal and genital ducts open into a cloaca. Heart 

 has auricle divided by a septum, and contains both pure and 

 impure blood ; the twisted conus has transverse rows of pocket 

 valves and a longitudinal valve. The blood system shows 

 modification towards the Amphibian type, e.g., the left auricle 

 receives blood direct from lung by two pulmonary veins, 

 and there is an inferior vena cava. Brain resembles that of 

 Amphibians ; and there is an optic chiasma. Ova of moderate 

 size ; segmentation total (holoblastic), but unequal. Primitive 

 fishes, survivors of an ancient race, sluggish in habit, in many 

 ways (e.g., their clambering movements) strongly suggestive 

 of the newt, and illustrating " discontinuous distribution." 

 (See Part II., page 137.) 



Examples, Ceratodus (the Australian Lung-fish, or 

 " Burnett Salmon "). Blunt fish- 

 like shape. Acutely lobate paired fins 

 (typical archipterygia, see page 194). 



