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small. There are ten pairs of cranial nerves. The olfactory 

 organ is paired,* and rarely communicates with the buccal 

 cavity. The hypophysis is not connected with the nasal 

 organ.* 



The ear consists of an otocyst with three semicircular 

 canals,* and both utricle and saccule contain otoliths ; 

 there is no middle ear (tympanum). The longitudinal 

 lateral muscles of the body are divided into segments or 

 myotomes. In certain fishes portions of the muscular 

 system are modified as electric organs (e.g., skate, in lateral 

 muscles of tail). An air-bladder (hydrostatic, or also 

 functioning as a lung) is frequently developed. The heart 

 consists of a sinus venosus, an auricle, a ventricle and conus 

 arteriosus. In Teleosteans the conus is absent, and the 

 dilated base of the ventral aorta forms a bulbus arteriosus. 

 Except in Dipnoi, in which the auricle is practically double, 

 the heart contains impure or venous blood only ; the purified 

 blood flows direct from gills to body. Besides the char- 

 acteristic posterior cardinal veins, there is also, in Dipnoi, 

 an inferior vena cava. The ventral anus is in front of the 

 urinary and genital openings ; but when these open into 

 the rectum, the common exit is a cloaca. The functional 

 kidney of the adult is the opisthonephros. The sexes are 

 usually separate. Most, fishes are oviparous ; the ova are 

 small (except Elasmobranchs). The " spawn " is generally 

 fertilised in the water, and usually there is a prolonged 

 larva state. 



Mention the chief Features which distinguish Fishes from 

 Cyclostomata. 



Those features marked * in previous answer. Fishes 

 have also a spleen, a renal portal system, and genital ducts. 



Explain the following terms : protocercal, diphy cereal, hetero- 

 cercal, homocercal. 



These terms indicate the formation of the tail in Fishes. 



Protocercal or diphycercal, i.e., end of notochord or 

 vertebral column is straight, and the caudal fin is vertically 

 symmetrical around it (e.g., Dipnoi, Eel). 



Heterocercal, i.e., end of vertebral column is bent up 

 dorsalwards ; and the caudal fin, along with a ventral 

 enlargement, forms a secondary bifid tail fin with unequal 

 lobes (.e.g., Dogfish, and other Elasmobranchs). 



Homocercal, i.e., end of vertebral column is at first bent 

 up ; but later it atrophies, and the ventral portion of the 

 caudal fin is enlarged to form an outwardly symmetrical 

 bifid tail fin with equal lobes (e.g., most Teleosteans). 



