634 



PHYLUM CHORDATA : CLASS PISCES — FISHES 



projecting out of the gill-clefts, so-called external gills. 

 They are really elongated internal gills. Elasmobranchs 

 retain more embryonic features, e.g. the naso-buccal 

 groove and auditory opening, than other fishes. 



Order i. Plagiostomi or Selachii 



With transverse ventral mouth, pre-oral rostrum, uniserial paired 

 fins, claspers, heterocercal tail, usually five pairs of open gill-clefts. 



Subdivisions. — (i) The older Selachoidei, with approximately 

 cylindrical bodies and lateral gill-openings, as in shark and dogfish ; 

 (2) the more modified Batoidei, with flattened bodies, ventral gill- 

 openings, and pectoral fins joined to the head, as in skates or rays. 



Mustelus, Carcharias, Squalus, Torpedo, Acanthias, and others, are 



Fig. 362. — Lateral view of dogfish {Scyllium catulus). 



Note ventral mouth with naso-buccal groove, heterocercal tail, and 

 unpaired fins, gs., Gill-slits ; pc, pectoral fins ; pv., pelvic 

 fins. 



viviparous ; Raja, Scyllium, Cestracion, and others, are oviparous. In 

 most species of Mustelus there is a placenta-like connection between 

 the yolk-sac of the embryo and the uterus of the mother. In several 

 viviparous genera long filaments are developed from the inner surface 

 of the uterus which secrete a nutritive fluid. In some cases the 

 nutriment seems to be afforded by degeneration of the uterine wall. 

 In Acanthias vulgaris there is no nutritive material, and the young 

 are unattached. This is intermediate between oviparous and specialised 

 placental conditions. Zygcena has a pecuHar hammer-like head expan- 

 sion ; Selache reaches a length of 40 ft. ; Pristis has the snout prolonged 

 in a tooth-bearing saw ; Toipedo has a powerful electric organ. The 

 Greenland Shark {Lcemargus borealis) is unique in having small eggs, 

 without egg-cases, perhaps fertilised in the water. In the eel-like 

 deep-water Japanese Shark (Chlamydoselachus) the mouth is anterior, 

 the nostrils lateral, the vertebral column is imperfectly segmented. 

 there is a slight opercular fold, and there are six pairs of gill-openings 

 and arches. In the large viviparous Notidanidaj, e.g. Hcxanchus (six 

 gills) and Heptanchus (seven gills), the mouth is almost inferior, the 

 vertebral column is imperfectly segmented with persistent notochord. 



