,.A THE DOGFISH. PHYLUM CHORDATA 



November and lasts at least until the following July ; sexual 

 activities take place in shallow water. 



The eggs of Scyllium are laid in flat, oblong, brown shells 

 (or mermaids' purses), the angles of which are prolonged into 

 tapering tendrils, which twine round seaweeds and thus anchor 

 tiie Q)^g (Fig. 253). Protected by the shell, the young dogfish 

 develops slowlv at the expense of the yolk, which comes to be 

 contained in a sac attached to its belly. At one stage long, vascular 

 threads project from the gill clefts of the httle fish. These are 

 the so-called external gills, but they are covered with endoderm 

 and thus differ from the true external gills of the tadpole. 

 Acanthias is viviparous— that is, the eggs hatch internally. The 

 embryo is nourished through vascular and secretory ridges on 

 the walls of the oviduct. 



NERVOUS SYSTEM 



The spinal cord of the dogfish has the general features of that 

 of a vertebrate as seen in a mammal (p. 458) and will not be 

 described here. The brain also has the same parts, but differs 

 from that of a mammal in not being bent; the shape and relation- 

 ship of the parts are shown in Figs. 254 and 255. The anterior 

 cerebrum is not divided into the two hemispheres of the mammal, 

 but has two internal cavities or lateral ventricles. These extend 

 into the olfactory lobes, which are large and extend forwards 

 from the anterior corners of the cerebrum to press against the 

 olfactory organs, which are covered by the olfactory capsules. 

 Behind the cerebrum comes a long thalamencephalon, with a 

 thin roof from the hinder part of which rises the pineal stalk ; 

 this runs forward to end in a pineal body below the anterior 

 fontanelle. From the floor of the thalamencephalon the infundib- 

 ulum extends downwards ; at its sides are a pair of inferior lobes, 

 and behind these it forms the saccus vasculosus and the nervous 

 part of the pituitary body (p. 367). The cavity of the thalamen- 

 cephalon is the third ventricle. Behind the thalamencephalon is 

 the mid-brain, with two large dorsal optic lobes, and behind this 

 again a large cerebellum. This is succeeded by the medulla 

 oblongata, with thin roof and a pair of restiform bodies projecting 

 forward from it alongside the cerebellum. The cavity of the 

 cerebellum and medulla is the fourth ventricle ; this is connected 

 to the third ventricle by a narrow passage in the mid-brain, the 



