iv. s SKELETON OF LAMPREYS 87 



mas and thus eventually the subcutaneous connective tissue. Within 

 this connective tissue there develop certain irregular cartilaginous 

 thickenings that are of special interest because they may be compared 

 with vertebrae, perhaps with the basi-dorsal element (p. 132). They 

 lie on either side of the spinal cord (Fig. 50), that is to say, above the 

 notochord, and consist either of one nodule on each side of the seg- 

 ment, through the middle of which the 

 ventral nerve-root emerges, or of two 

 separate nodules, with the nerve between 

 them. Rods of cartilage extend dorsally 

 and ventrally into the fins, but are not 

 attached to the 'vertebrae'. 



The lamprey skull shows even in the 

 adult the basic arrangement found only 

 in the embryo of higher vertebrates. The 

 floor is formed of paired parachordals on 

 either side of the notochord and in front 

 of this paired trabeculae. Attached to this 

 base is a series of incomplete cartilagi- 

 nous boxes surrounding the brain and 

 organs of special sense (Fig. 51). To 

 this skull is attached the skeleton that 

 supports the sucker and gills. The 

 arrangement of the skull differs consi- 

 derably from that of later vertebrates. 

 The cranium has a floor around the end 

 of the notochord, and in front of this 

 there is a hole containing the pituitary 

 gland. The side walls are strong but the roof is composed only of 

 a tough membranous fibro-cartilage. The auditory capsules are 

 compact boxes surrounding the auditory organs at the sides. The 

 olfactory capsule, imperfectly paired, is also almost detached from 

 the cranium. Other ridges of cartilage lie below the eyes and there is 

 a complex support for the sucker. 



The skeleton of the branchial region consists of a system of vertical 

 plates between the gill-slits, joined by horizontal bars above and 

 below them. This cartilage lies outside the muscles and nerves and is 

 therefore difficult to compare with the branchial skeleton of higher 

 fishes, which lies in the wall of the pharynx. The elastic action of the 

 cartilages produces the movement of inspiration. A backward exten- 

 sion of the branchial basket forms a box surrounding the heart. 



Fig. 51. Dorsal view of skull of 



Petromyzon. 



Lettering as Fig. 50. /, olfactory 



nerve; /. hole in roof of cranium. 



(After Parker.) 



