VERTEBRATE ANIMALS 205 



The lesser spotted dogfish is found in shoals round the 

 coast. It is a voracious feeder, living on other fish, on cuttlefish, 

 octopus, crustaceans, etc., and being powerful and swift does 

 considerable damage to ordinary edible fish. It is marketed as 

 food, but is much coarser eating than the other, bony fish, and is 

 consequently not widely used. Before considering it in detail it 

 will be well to glance at some of the main characteristics of a Verte- 

 brate which are common to the dogfish, frog and rabbit, in the types 

 we have to study, and also, of course, to ourselves. 



Like the earthworm, all the Craniates are ccelomate 

 metazoa, and are primitively segmented, and, although this meta- 

 merism becomes partially obscured in the adult, traces of it are 

 always to be found. The body is generally to be marked off into, at 

 any rate, a head, a trunk and a tail. They usually lay eggs, i.e. 

 are Oviparous, even though these may be hatched inside the mother 

 as in some Reptiles, and an ally of the dogfish Acanthias, a condition 

 termed Ovo-viviparous. In the higher groups of the mammals we 

 meet with a truly Viviparous condition in which the young are 

 intimately attached to the mother at a very early stage, and are 

 brought forth alive in a fairly advanced stage of development. 



The first and, perhaps, most important feature, however, 

 concerns the skeleton which, in the embryo, is furnished by a very 

 characteristic structure, the Notochord or Chorda dorsalis. This 

 consists of an elastic rod of very highly modified entodermal cells 

 that typically come from the dorsal side of the gut, and it is the 

 presence of this same striking tissue in the ACRANIA that leads to 

 their being grouped with the CRANIATA in the one great phylum 

 CHORDATA. In most craniates the notochord is replaced * almost or 

 completely by the Vertebral Column, a mesodermal structure which 

 comes to form the main supporting axis of the body. As we have 

 seen in the frog, this consists of a number of separate pieces or 

 vertebrae movably articulated one with another. Not only is it a 

 support for the animal as a whole, but it also bears on its dorsal side 

 a canal, the Vertebral Canal, for the protection of the spinal cord. 

 Its anterior end is modified to form a skull with which are associated 

 upper and lower jaws.* Also more or less closely connected with 

 the vertebral column are two pairs of limbs,* which in the fish form 

 the fins, and in higher vertebrates the legs, though the anterior 

 pair are sometimes specially modified to form arms or organs of flight, 

 termed wings. 



The second fundamental characteristic of a Vertebrate is its 

 central nervous system, which is tubular in form and situated 



* A somewhat aberrant group of Vertebrates, the Cyclostomes (Lampreys 

 and Hagfish), are not taken into account. 



