PHYLUM CHORDATA 115 



-\ stem and notochord, and a ventral portion, the lateral plate, 

 bounding the mesenteron. The vertebral plate undergoes meta- 

 meric segmentation, becoming divided into a row of squarish 

 masses, the protovertebrce or mesodemnal segments (B, pr. v.) : the 

 lateral plate splits into two layers, a somatic (som.) adherent 

 to the ectoderm, a splanchnic (sp/.) to the endoderm. The space 

 between the two is the coelome (ccel.), which is thus a schizoccele or 

 cavity holloAved out of the mesoderm and is at no stage in com- 

 munication with the mesenteron, like the coelomic pouches of 

 Amphioxus. _ A dorsal offshoot of the coelome (ccel') may pass into 

 each protovertebra, but such an arrangement is temporary. From 

 the dorsal portions of the protovertebras the myomeres are formed, 

 from their ventral portions the vertebrae. 



The development of the principal organs has been described, in 

 general terms, in the preceding account of the organs themselves : 

 it will be convenient to defer further consideration of this subject 

 until we come to deal with the development of the various types 

 of Craniata, and with the embryological characteristics of the 

 classes and sub-classes. 



Distinctive characters.- -The Craniata may be denned as 

 Vertebrata in which the notochord is not continued to the end of the 

 snout, but stops short beneath the fore-brain, some distance from its 

 .anterior end. A skull is always present, and there are usually paired 

 limbs. The ectoderm is many-layered and is never ciliated in the 

 adult, and only rarely in the larva. The pharynx is of moderate 

 dimensions, and is perforated by not more than seven pairs of 

 gill-slits. There is no atrium. The liver is large, massive, and not 

 obviously tubular. There is a muscular chambered heart, and the 

 blood contains red corpuscles. The nephridia (mesonephridia) unite 

 to form large paired kidneys and open into ducts which discharge 

 into or near the posterior end of the intestine. The brain is com- 

 plex, and there are at least ten pairs of cerebral nerves : the spinal 

 nerves are, except in Cyclostomes, formed by the. union of dorsal 

 and ventral roots. Paired eyes of great complexity, derived in 

 part from the brain, are present, and there is a pair of auditory 

 organs. There is a single pair of gonads, and the reproductive 

 products are usually discharged by ducts derived from the nephri- 

 dial system. There is never a typical invaginate gastrula, and 

 the mesoderm arises in the form of paired longitudinal bands which 

 subsequently become segmented. The coelome is a schizoccele. 



CLASS I CYCLOSTOMATA. 



The Cyclostomata, or Lampreys and Hags, are eel-like Fishes, 

 distinguished from all other Craniata by the possession of a 

 suctorial mouth devoid of functional jaws, by the single olfactory 

 organ, and by the absence of lateral appendages or paired fins. 



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