INV-ERTEBRATE ZOOLOGY. 15 



Cladus III. Cephalochorda. 



Elongated fish-like marine forms, withoiit head, brain, or sknll. Notochord 

 persistant, extending through entire body from tix^ to tip. Mouth near ante- 

 rior end. Pharynx perforated by paired gill slits, which open, not to the ex- 

 terior, but into a space formed by integumental folds, the peri-branchial cav- 

 ity. This communicates with the exterior through a median opening, anterior 

 to the arms, the atriopore. 



The nervous system consists of a dorsal cord lying upon the notochord. It 

 possesses a central lumen which enlarges a little at the anterior end. The 

 sense organs are a set of cilia about the mouth, a median olfactory pit and a 

 median pigment-fleck lying upon the anterior end of the nerve cord. 



The reproductive glands are a series of pairs of follicles projecting into the 

 peri-branchial cavity. The germ cells become free by rupture of the follicu- 

 lar walls, and later pass into the water through the moiith or the atriopore. 

 The excretory organs consist of paired nephridia in the branchial region, 

 which pass from the true c-oelom into the peri-branchial ca\aty. 



Cladus IV. Vertebrata. 



The main group of Chordata. characterized by the possession of a head and 

 two pairs of lateral fins, which maj' become modified in various ways. The 

 anterior portion of the central nervous system becomes enlarged to form a 

 brain, which consists of five vesicles, and bears three pairs of sense-capsules, 

 nose, eye and ear. 



The notochord. which ends antei'iorly at about the level of the ear-capsules, 

 is cartilaginous, in some stage at least, but in the higher forms is reinforced 

 by segmentally arranged osseous arches and other elements alternating vnth 

 the mesodermic somites, which may entirely replace the original structure 

 (vertebral column.) The brain and sense capsules are protected by a cranium, 

 which consists primarily of two pairs of cartilaginous elements, to which may 

 be added osseous tissues. Visceral arches always appear, but in the higher 

 terrestrial forms, only show their primitive relationship in larval or embryonic 

 life, and later undergo modification and reduction. The germ-glands are 

 surface-gonads, i. e. localized portions of the coelomic mesoderm. 



The excretory organs develop as paired nephridia, appearing in several 

 series ; these are consolidated to form kidneys, or utilized to convey the germ- 

 cells. 



