THE SKULL OF FISHES. 



71 



therefore, may be said to be composed of tlie primitive continuous 

 fibro-gelatinous basis of the vertebral bodies, and of the membrane 

 which is represented by our ' dura mater,' without the superaddition 

 of cartihxginous or osseous coverings. 



But if we were to limit our view of the skull of the Dranchiostoma 

 by this pinmitive embryonic condition of the cranium proper, we 

 should have an incomplete idea of it. A large, jointed, cartilaginous 

 htemal arch (^Jig. 46. //) extends on each side, from below the cranial end 

 of the chorda dorsalis, downwards and backwards to the orifice of the 

 pharynx ; this represents the labial arch of higher Myxiuoids, and it 

 supports the jointed slender oral filaments, which may be regarded 

 as a continued representation, in the Vertebrate series, of the cephalic 

 tentacula of the Cephalopods. It is the sole chondrified part of the 

 skeleton in the Branchiostoma, a fact which must be borne in mind 

 if we would avoid the common error of supposing the neural vei'- 

 tebral column to be the first and only rudiment of an internal 

 skeleton in the lower Vertebrata. 



Before proceeding to the next stage at which cranial development 

 is arrested in the ascending series of Vertebrata, I may briefly de- 

 scribe the form under which the cartilaginous tissue is superinduced 

 upon the fibrous brain-sac in osseous fishes, according to the obser- 

 vation of M. Vogt on the embryo of one of the Salmonidaj ( Core- 

 gonus, XXII. tom. i. p. 3.). The chorda dorsalis advances as far as 

 the pituitary sac, or ' hypophysis cerebri,' where it terminates in 

 a point ; cartilage is developed on each side of the chorda, forming a 

 thick occipito-sphenoidal mass*, which extends outwards, and en- 

 velopes the sac of the internal ear, forming the ear-ball or acoustic cap- 

 sule. The cartilage rises a little way upon the lateral walls of the cra- 

 nium, and is there insensibly lost in the primitive cranial membrane. 

 At the end of the chorda, the basal cartilages diverge, surround the 

 pituitary vesicle, and meet, in fi-ont of it, to join or be expanded in 

 the ^:>resp/^c«o^c? plate f : these arches I term " sphenoidal.'''' \ (Jig. 24.) 



Base of skull, Atmnocctc, Miillfi'. 



Side view of skull, Amntocclc, Miilkr. 



* Flaque imchale, Vogt; Knochcrne basis cranii, Miillcr, xxi. 

 t Plaque faciale, Vogt; Gaumenplatte, Miillcr. 

 ] Anses latcraks, Vogt ; F lag el- for sal ze basis cranii, Miillcr. 

 F 4 



