XIII PHYLUM CHORDATA. 123 



pronounced, and that more particularly in the lower groups. In 

 the head there is great specialisation in co-ordination with the 

 presence in this region of the brain, the chief organs of special sense, 

 and the mouth and jaws ; so that, though there are indications 

 of metamerism of various parts, it is only by the study of 

 development that it is possible to interpret the structure of the 

 head in terms of a metameric segmentation which becomes much 

 modified and disguised in the adult animal. When the develop- 

 ment is followed out, it becomes evident that, as in the 

 Arthropoda, the head in Craniata is formed as a result of a 

 process of fusion between a number of metameres, the individual- 

 ity of which is quite evident in early stages, more particularly 

 among lower forms, being most pronounced in the region behind 

 the auditory capsules. 



Distinctive Characters. — The Craniata may be defined as 

 Euchorda 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 (except in some Cyclostomes). The gill-pouches 

 do not open into an atrium. The liver is large, massive, 

 and not obviously tubular. There is a muscular chambered heart, 

 and the blood contains red corpuscles. The renal tubules 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 typically a single pair of gonads, and the re- 

 productive products are usually discharged by ducts derived from 

 the renal 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 nearly always developed as a schizocoele. 



CLASS I-CYCIOSTOMATA. 



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

 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. 



