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SUB-PHYLUM CRANIATA. 

 Structure and Development of the Craniata. 



Describe the External Features of the Body. 



The body is elongated, more or less rounded and bulky, 

 and bilaterally symmetrical ; and it has distinct regions, 

 namely, head, neck (frequently), trunk (thorax and abdomen), 

 and tail (an appendage in higher Craniates). The mouth is 

 transverse and anterior ; and near it are the paired nostrils 

 (single in Cyclostomata). At the sides of the head are the 

 paired eyes ; and behind them are the ears, and the gill- 

 slits or branchial clefts (in the neck region). The anus 

 is ventral, at or about the end of the trunk, and near it 

 are the urinary and genital openings (frequently combined). 

 Commonly there is only one opening, that of the cloacal 

 chamber, within which are the anus and urino-genital 

 apertures. 



There is a dorsal unpaired fin which extends round the 

 tail in certain Fishes and Amphibians, and which in most 

 Fishes is broken up into separate fins. Fishes have also 

 paired fins, representing limbs ; and there are two chief 

 types, namely, the primitive archiptergyium (e.g., Cera- 

 todus), which consists of a median segmented axis bearing 

 on each side supporting radials obliquely directed and 

 overlapped by a fringe of fin-rays, and the ichthyopterygium 

 (of most fishes), in which the radials arise from basal pieces. 

 In higher Craniates these fins are replaced by fore- and 

 hind-limbs, each of which has, typically, five digits (penta- 

 dactyle limb or cheiropterygium). 



Give a, Description of the Structure of the Integument or Skin, 

 and state its Functions. 



The skin consists of an outer epidermis (modified ecto- 

 derm) and an underlying dermis or cutis (connective tissue 

 formed from mesenchyme). The epidermis has several 

 layers, the lowest being the actively growing stratum 

 malpighii which forms and renews the upper layers (stratum 

 corneum). 



The epidermis of fishes has gland cells, which secrete 

 mucus for reducing friction between the body and the 

 water. The preen-gland of birds (for oiling the' pluin;iir<0- 

 also feathers and hairs are formed by the epidermis. Pig- 

 ment (to prevent penetration of light), glands fcr keeping 

 the skin moist (e.g., frog), scales (of fishes and reptiles), 



