168 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



into folds which interlock in corresponding folds of the oviduct, so that a 

 kind of placenta is formed i.e., an embryonic structure by which the blood- 

 systems of parent and embryo are brought into close relation. Scyllium, 

 therefore, is oviparous, a term applied to cases where most or all of the 

 development takes place external to the body other dogfish are viviparous. 



8. The most obvious part of the muscular system is a great 

 lateral mass extending along each side of the body from the 

 neighbourhood of the spiracle, and segmented into a succession 

 of myomeres, with zigzag boundaries. The lateral muscles are 

 also divisible into dorsal and ventral sections along the boundary 

 between which runs the lateral line. These muscles effect 

 swimming movements, during which the body is not merely 

 bent from side to side but thrown into sinuous curves. There 

 are also special muscles for moving the fins, lower jaw, &c. 



9. The nervous system (Fig. 48) consists of (1) cerebro-spinal 

 axis, (2) cranio-spinal nerves, and (3) sympathetic system. 



(1) The cerebro-spinal axis is a thick- walled tube invested 

 in a delicate vascular membrane (pia mater) and contained in 

 the neural canal of the skull and vertebral column. The canal 

 is lined by a firm membrane (dura mater) between which and 

 the pia mater there is a large lymph-space. The front end of 

 the cerebro-spinal axis is dilated into a brain, lying within the 

 cranium and a spinal cord, running along the spinal canal above 

 the vertebral centra. The cavity of the neural tube constitutes an 

 extremely small central canal in the spinal canal and larger spaces 

 known as ventricles in the brain. 



The brain at an early stage exhibits three successive swellings, 

 the anterior, middle, and posterior cerebral vesicles. These, 

 together with outgrowths from them, become the fore-, mid-, 

 and hind-brains of the adult. 



The fore-brain consists of a central thalamencephalon, with a 

 large antero-dorsal outgrowth, the prosencephalon, with which 

 are connected two olfactory lobes. 



The thalamencephalon is somewhat cylindrical and contains a 

 large cavity (3rd ventricle) with thin roof but thickened floor 

 and side walls. The floor is produced downwards into a pro- 

 jection (infundibulum) in which are two oval swellings (lobi 

 inferiores). Two structures of non-nervous nature are connected 

 with the thalamencephalon. They are known as the pineal and 

 pituitary bodies. The pineal body is a stalk-like structure with 



