PISCES. 417 



nected with the cartilaginous bars of the claspers (ptery go-podia) in the 

 male and their rudimentary representatives in the female. In all other 

 Fish a metapterygium only is present, very broad and even secondarily 

 segmented in chondrostean Ganoidei. The two metapterygia are also 

 independent in them, except in Scaphirhynchus, where, as in bony Ganoidei 

 and Teleostei, the two are united medianly with more or less completeness. 

 The number of radialia is great in chondrostean Ganoidei. They have 

 two rows ; so too Polypterus ; but other Ganoidei are reduced to a single 

 row, usually inconspicuous and few. The fin-rays resemble those of the 

 fore-limb. The Dipnoi differ from the preceding in having a limb com- 

 posed of an axial series of cartilaginous pieces ranged end to end. These 

 in Ceratodus from the second onwards bear a bilateral series of jointed 

 cartilaginous rods diminishing in length to the apex of the limb, and 

 continued outwards into horny fin-rays. Protopterus amphibius has a uni- 

 lateral series of such rods, always very small, absent in P. annectens and 

 Lepidosiren. Their limb is long and linear. 



It may be noted that in some Elasmobranchii there are radialia, 

 always few in number, on the posterior or inner edge of the metaptery- 

 gium. They are probably secondary in origin. The ventral fins in 

 all Fish have an abdominal position, except in some Teleostei, where 

 they may shift forwards and become thoracic or even jugular in position 



(P- 83). 



The muscles of the trunk and tail retain their segmentation. A fibrous 

 septum in the median dorsal and ventral lines divides the musculature of 

 one side of the body from that of the other. Another separates the dorso- 

 from the ventro-lateral muscle-masses. The muscle fibres run longi- 

 tudinally from one myocomma (septum between two muscle segments or 

 myomeres) to the next. They are oblique in the ventro-lateral trunk 

 muscles of Elasmobranchii and Ganoidei, and the corresponding region in 

 Protopterus is divided into two oblique layers, as in Amphibia Urodela. 

 The electric and pseudo-electric organs are apparently modified muscular 

 tissue, except perhaps in Malapterurus (Siluroidei), a fish in which the 

 organ lies beneath the skin and external to the muscles, and forms a 

 nearly complete envelope round the trunk. The two organs in Torpedo 

 (Batoidet) lie on either side the head, and are supplied by a nerve from the 

 electric lobe of the medulla oblongata. In the Eel, Gymnotus, they replace 

 the greater part of the ventro-lateral muscles of the tail, as do the pseudo- 

 electric organs of Rays, Mormyrus, Gymnarchus among Teleostei Physostomi, 

 and they are supplied by spinal nerves. Each organ consists of a number 

 of long hexagonal capsules, placed dorso-ventrally in Torpedo, antero- 

 posteriorly in the remaining Fish, and containing each an electric plate, to 

 one surface of which the nerves are distributed, whilst the other is in 

 relation with a gelatinous substance. The nerves break up into fine 



E e 



