444 GENERAL ZOOLOGY 



The spinal cord gives off a pair of dorsal and ventral nerves to 

 each segment of the body, the dorsal and ventral nerves arising 

 independently from the cord. The blood circulatory system 

 consists of (1) a longitudinal vessel dorsal to the alimentary 

 canal and divided into right and left forks over the branchial 

 region; (2) a longitudinal vessel lying ventral to the alimentary 

 tract; and (3) lateral connections between the longitudinal 

 vessels, those passing between the gill clefts functioning as 

 branchial vessels. The blood is circulated by the contractions 

 of the anterior end of the ventral longitudinal vessel and parts 

 of other vessels. The blood moves forward in the ventral 

 vessel and backward in the dorsal one. 



The sexes are separate. All are marine. 



Example: Branchiostoma virginiae, one of the lancelets, also 

 known as Amphioxus, lies buried in the sand up to the mouth in 

 an upright position. About 5 cm. in length, it is found on the 

 southeastern Atlantic Coast. There are also numerous other 

 species. 

 SUBPHYLUM 4. VERTEBRATA. A notochord develops in the 

 embryo and, in some cases, persists as a rod-shaped structure in 

 the adult. In most of the fishes, including the sharks, and in 

 all higher vertebrates, it is partially or totally replaced by the 

 centra of the vertebral column. The skeleton of vertebrates is 

 an endoskeleton composed of cartilage or bone, or a combina- 

 tion of both. The integument consists of two well-defined 

 layers: the epidermis and the corium, supplemented by (1) 

 epidermal modifications such as the scales of reptiles, the feath- 

 ers of birds and the hair of mammals; and (2) modifications in 

 the corium, such as the scales of most fishes, and dermal bone of 

 certain reptiles and mammals. The body is fundamentally 

 bilaterally symmetrical and is made up of at least three body 

 parts: head, trunk, and tail (postanal region). There are 

 usually two pairs of locomotory appendages : the paired fins of 

 fishes and the fore- and hindlimbs of other forms. Paired 

 appendages are lacking in the lowest vertebrates (Cyclosto- 

 mata), some Amphibia (Apoda), snakes, and some lizards. 

 Dorsal, ventral, and caudal median unpaired fins are found in 

 aquatic adult and larval forms. The segmental character of 

 the vertebrate body is shown by the metameric arrangement 

 of the vertebrae, the nerves, and the body muscles. The body 

 cavity is a coelom in which the alimentary canal is attached by 



