xm PHYLUM CHORDATA 217 



which are in apposition so as to form a keel-like ridge. The 

 endoderm and mesoderm are formed as a result of a process of 

 infolding of the posterior edge of the blastoderm (Fig. 871). 



Fig. 871. — Longitudinal section of blastoderm of SalxuOj at about the stage represented in D 

 of Fig. S70. ec. ectoderm ; en+ms, infolding giving i-ise to endodei-m and mesoderm. 

 (Alter O. Hertwig.) 



Gradually the head and tail become free from the yolk, and at the 

 time of hatching the yolk-sac (I, y. s.) is a shoe-shaped body sessile 

 upon the ventral surface of the transparent embryo. 



2. — Distinctive Characters and Classification. 



The Teleostomi are Pisces in which the primary cranium is 

 always complicated by the addition of investing bones, of which 

 a pair of parietals and one of frontals above, and unpaired vomer 

 and parasphenoid below, are the most constant. The chondro- 

 cranium is always more or less ossiiied by replacing bones, including 

 a supraoccipital, and the upper and lower jaws are both bounded 

 by investing bones. The jaws are connected with the cranium 

 through the intermediation of a hyomandibular, which is probably 

 not homologous with the similarly named element of Elasmobranchs. 

 The dermal fin-rays are formed of bone, and are supported by 

 pterygiophores which may be either cartilaginous or bony, but 

 which always show a great reduction in number as compared with 

 the homologous structures in Elasmobranchs. The primary shoulder- 

 girdle is complicated by the addition of investing bones, of 

 which a large clavicle is the most constant. The pelvic 

 girdle is vestigial or absent. The pelvic fins usually undergo 

 a forward displacement, their position being either abdominal 

 i.e., between the anus and the pectoral region, or thoracic, i.e., in 

 the pectoral region, or jugidar, i.e., under the throat. A dermal 

 exoskeleton is usually present. The intestine may or may not 

 have a spiral valve ; the anus is distinct from, and placed in front 

 of, the urinary and genital apertures. The gills are covered by an 

 operculum supported by investing bones, and the interbranchial 

 septa are reduced or absent, so that the gill-filaments are partially 

 or wholly free ; the hyoidean gill is reduced or absent. The conus 



