566 The Animal Kingdom 



and passes into the atrial cavity. From the atrial cavity, the water passes 

 out of the body through the atrial pore. 



The circulatory system has no definite heart, but does have a series 

 of blood vessels in which blood circulates in much the same manner 

 as in vertebrates. In addition to the blood vessels there are a few open 

 blood spaces. The excretory system consists of ciliated nephridia dorsal 

 to the pharynx. They connect to the atrial cavity in a manner somewhat 

 similar to that of the annelids. A coelom is formed in the embryo, but be- 

 comes reduced in the adult. It is only seen around the intestine and in 

 these ciliated nephridia. Above the notochord is the dorsal tubular 

 nerve cord. It is slightly enlarged at the anterior end into a primitive 

 brain. This cerebral vesicle has an olfactory pit in its mid-dorsal portion, 

 two pairs of nerves, and a small eyespot. The cord gives ofif paired 

 nerves along its length to the myotomes. The sexes are separate ; the 

 gonads of each sex are located in two rows. The gametes pass to the out- 

 side through the atrial pore. Fertilization is external with the result- 

 ing free-swimming larva closely resembling the adult. 



Ordinarily amphioxus is found lying in a curved tube in the sand 

 with the anterior and posterior ends near the surface. Water is forced 

 into the pharynx and the small detritus is collected by the mucus of the 

 endostyle and hyperbranchial groove. These small forms are often lo- 

 cally quite abundant and probably form an important part of the diet of 

 larger fish. In the Orient, they are used for food for human beings. 



THE SUBPHYLUM VERTEBRATA 



By far the largest number of chordates are members of this sub- 

 phylum, the Vertebrata. The name of the subphylum is derived from 

 the vertebral column which is divided into individual vertebrae. This 

 feature shows a progressive development from the primitive vertebrates 

 where it is little more than a few cartilaginous bars to the well-developed 

 vertebrae of higher forms. This vertebral column becomes the main 

 supporting axis of the body, replacing the notochord which is retained 

 only as cartilaginous pads between the individual vertebrae. 



Other distinguishing features of the vertebrates are the well-de- 

 veloped brain at the anterior end of the dorsal tubular nervous system, 

 the protecting cranium, the extraordinary degree of cephalization, the 

 chambered heart with associated blood vessels, the internal, living jointed 

 skeleton, the stratified epithelium forming the integument, the two pairs 

 of appendages, the postanal tail, and the excretory system consisting of 

 paired kidneys. 



