CYCLOSTOMATA 425 



point the case is encircled by an opercular groove, which enables 

 the polar portion to be thrown off like a cap at the time of 

 hatching, so as to allow the young Bdellostoma to make its 

 escape. The large size of the egg, which almost completely fills 

 the cavity of the egg-case, is due to the fact that it consists 

 mainly of food yolk, the germinal protoplasm containing the 

 nucleus forming only a small hillock near the inner extremity of 

 the micropyle. Bdettostoma spawns during the greater part of 



FIG. 242. Embryo of Bdellostoma stouti near the time of hatching. 

 (From Bashford Dean.) 



the year, but chiefly in the early summer, and probably about 

 20 eggs are deposited at one time, generally on a shelly or 

 rocky bottom. After deposition the eggs become connected 

 together in long chains or clusters by the interlocking of their 

 polar hooks. Fertilisation takes place after extrusion, and the 

 segmentation is meroblastic and discoidal, much as in Teleosts. 

 The embryo completes its development within the egg, and when 

 hatched it is a miniature of the adult. 



Order II. Petromyzontes. 



In the Lampreys there is a large suctorial buccal funnel 

 leading behind and above into the mouth, which is supported 

 by special cartilages, and furnished with a marginal fringe of 

 small cirri. Numerous horny teeth are present on the inner 

 surface of the funnel as well as on. the tongue. The naso-pitui- 

 tary involution forms a caecum and does not communicate with 

 the mouth. The gill-sacs, seven in number, open externally by 

 separate orifices, but internally they open into a median branchial 

 canal, situated below the oesophagus and opening into the mouth 

 in front. There is a well-developed branchial basket. I)m>al 

 arcualia are present throughout the precaudal as well as in the 

 caudal region. A rudimentary spiral valve is present. The 

 brain consists of parts usually present in other Craniates, includ- 

 ing cerebral hemispheres and a cerebellum. The auditory organ 



