84 DESCRIPTIONS OF PREPARATIONS. 



roof of the cranium, the right opercular apparatus and right side of the 

 body walls. 



There are four divisions of the brain visible. They do not overlap one 

 the other, nor do they fill the cranial cavity as in the young frog. The first 

 division forms the olfactory lobes or rhinencephala from which the olfactory 

 nerves may be seen passing forward : the second division the cerebral 

 hemispheres or prosencephala : the third the optic lobes, corpora bigemina 

 or mesencephalon, the largest of all the divisions in Teleostei \ and the 

 fourth the cerebellum which is subglobular and, unlike the preceding parts 

 of the brain, unpaired. 



The removal of the operculum on the right side displays the four gill- 

 arches with their dotible series of gill-filaments, arranged like the teeth of a 

 comb. Each arch is hence said to be bi-pectinate. Internally to the last 

 or fourth gill-arch, and anteriorly to the liver from which in the natural 

 state it is separated by a fibrous septum, lie the heart and ventral aorta. 

 The aorta has a distinct bulb or swelling where it springs from the 

 ventricle : this, the most muscular part of the heart, is in contact with the 

 body walls ventrally while the thin auricle is placed dorsally to it. The 

 liver is large and imperfectly divided into three lobes of which the left is 

 not exposed to view. The single ovary lies posteriorly to the liver and 

 being ripe occupies the greater portion of the abdominal cavity, having 

 displaced the remaining viscera. The anterior three-quarters of its right 

 side have been removed, but owing to the extreme state of distension of 

 the organ, it is not possible to make out the transverse ovigerous lamellae 

 which cross its interior. The commencement of the duodenum with one 

 the shortest of the three pyloric appendages, may be seen between the liver 

 and ovary. The two other appendages, the stomach,! and the loop of 

 intestine containing the spleen in its concavity, are all alike hidden on the 

 left side of the body. The gall-bladder has been displaced upwards and to 

 the right. It is lying on the under surface of the liver in a depression, the 

 homologue of the fossa cystis felleae of man, with which it does not in this fish 

 usually come into relation save when the ovary is in a state of turgescence. 



The terminal portion of the intestine and the rectum pass with a 

 straight course down the middle line of the body to the anus. This aper- 

 ture, into which a black bristle is inserted, is superficial in Cyclostomi, in 

 Teleostei and Ganoidei, and it is placed in front of the genito-urinary 

 depression, clearly visible here behind it. The air-bladder lies dorsally to 

 the ovary, and between its upper end and the liver posteriorly, and the 

 fourth gill arch anteriorly is a gland (really paired) the homologue of the 

 thymus. Bands of yellow-coloured fat in a state of atrophy correlated with 

 the hypertrophy of the ovary, are to be seen contained in the peritoneal 

 lamellae which unite the intestine to the ventral surface of the ovary as well 

 as along the outer or attached edge of the air-bladder. 



