66 



LECTURE III. 



fishes, by the numerous centres of ossification, from which shoot out 

 bony rays affording additional strength to many of the intermuscular 

 aponeuroses : some of these supernumerary or intercalary ossicles 

 belong to the endo-skeleton, but most of them to the exo-skeleton. 

 In the former system of bones may be ranked those spines which are 

 attached to, or near to, the heads of the ribs, and extend upwards, out- 

 wards, and backwards, between the dorsal and lateral masses of muscles : 

 these are the ' diverging appendages ' of the abdominal ribs 

 {Jig. 17. ip, Jiff. 23. pi a), and may be termed ' epipleural spines ; ' 

 though they sometimes pass gradually, as the vertebrae approach 

 the tail, from the rib upon the parapophysis, 

 and even in the posterior abdominal vertebras 

 (e. g. Jlolocentrmn), upon the bodies and neural 

 arches. They are the " obere rippe " of 

 Meckel, and at the fore-part of the abdomen, 

 in Polypterus, the epipleural spines are stronger 

 than the ribs themselves. The spinous appen- 

 dages are remarkably developed in the Halecoid 

 fishes, (Salmon and Herring,) in the Mackerel- 

 tribe, and the Dolphin {Cori/ph(B?ia). In our 

 common herring you will find them attached 

 not only to the ribs (Jig. 23. pi, a), but also di- 

 verging from the parapophyses (pa), and the 

 neurapophyses (na), and the vertebra is further 

 complicated by dermal bones, those on the under surface of the 

 abdomen (dk) being connected, like the scutes of sei'pents, with the 

 lower ends of the ribs (pi). 



The very distinct histological condition of the endo- and exo- 

 skeleton of the Sturgeon (Jig. 43.), shows clearly the nature of 

 those spines (Jig. 19. d?i, dn), whicli form, in osseous fishes, a second 

 row, of greater or less extent, above the true neural spines, and sup- 

 port the dorsal fins. Thus, in Accipenser Ratzburgii, twelve of the 

 hard enamelled calcareous plates (ganoid scales) along the mid-line of 

 the back, send upwards and backwards a moderately long spine : the 

 series is then continued in a cartilaginous state to support the dorsal fin. 

 In the Polypterus sixteen accessary bones, in the form of longer 

 and sharjicr spines, are extended over thrice as many vertebrae : 

 and each dermal spine supports a membrane, strengthened at its upper 

 part by four or five branched and jointed rays. From the base of 

 the dermal spines, other spines (Jig. 19. in, in) usually shoot down- 

 wards, into the intervals of the neural spines : these inverted spines 

 may be the homologues of the wedge-shaped interneural pieces before 

 noticed in the vertebra3 of Sharks, and may well retain that name in 

 the osseous fishes. Sometimes they are double, as in the Flat-fish 



Abdominal vertebra, 

 Herring {Clupea). 



