XVI CYCLOSTOMATA 425 
point the case is encircled by an opercular groove, which enables 
the polar portion to be thrown off lke a cap at the time of 
hatching, so as to allow the young JSdellostoma 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. Sdellostoma spawns during the greater part of 
hei 
Fic. 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. Dorsal 
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 
