xiir PHYLUM CHORDATA 119 



It is in the trunk region that the metamerism is most strongly 

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

 the head there is great specialisation in cc -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 development 

 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 individuality 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 denned a 

 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 complex, 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 reproductive 

 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 schizocoale. 



CLASS I-CYCLOSTOMATA. 



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. 



1. EXAMPLE OF THE CLASS. THE LAMPREY (Petromyzon). 



Three species of Lamprey are common in the Northern Hemi- 

 sphere : the Sea-lamprey (P. marinus), which attains a length of a 



