12 CHORDATE ANATOMY 



floor of the intestine just behind the pharynx and extends forward to the 

 left below the pharynx. The intestine is straight. 



The coelom, considerably reduced in size in the region of the pharynx, 

 extends posteriorly to the region of the anus. Ninety pairs of nephridia, 

 limited to the gill-region, open into the atrial cavity. The solenocytes 

 attached to the nephridia are specialized excretory cells which strikingly 

 resemble those of annelids. Nephrostomes are absent. (Fig. 263) 



Sexes are separate. Two dozen or more gonadic sacs surrounded by 

 the peritoneum project into the atrial cavity. Except for the absence of 

 a heart, the circulatory system resembles that of fishes, but the blood 

 contains few blood corpuscles. 



The nervous system, as in vertebrates, is tubular and dorsal. The 

 brain is a simple vesicle, which may possibly be compared with the fore- 

 brain vesicle of vertebrates. The nerves are of two kinds, dorsal (sensory 

 and motor) and ventral (motor). The former pass directly to the skin 

 and to visceral muscles by way of the myocommata. Dorsal and ventral 

 nerves do not unite. Sympathetic cells and fibers are not segregated to 

 form a sympathetic system. 



Sense organs comparable with those of vertebrates are wanting. 

 A median dorsal pit at the anterior end of the brain is mistakenly spoken 

 of as the olfactory pit. A pigment spot on the brain is likewise somewhat 

 uncritically called the cerebral eye. Amphioxus is, however, very sensi- 

 tive to light. There is no ear. 



Sub-Phylum Vertebrata (Craniota) 



The vertebrates or craniotes are chordates with a vertebral column 

 and a brain-case. The evolution and perfection of a light and strong 

 endoskeleton has been an important factor in making the vertebrates 

 masters of the world. Exoskeletal structures also appear, as among the 

 invertebrates, but only exceptionally are heavy enough to interfere with 

 the activity of the animal. A many-layered epidermis with various 

 appendages enables the vertebrates to withstand successfully the vicissi- 

 tudes of weather met by land animals. In correlation with their activity, 

 senses multiply and become acute and the brain is much enlarged. The 

 original metamerism characteristic of lower vertebrates becomes much 

 obscured in the higher. The heart is ventral and may be either two, 

 three, or four-chambered. 



Some 25,000 species are known. 



Vertebrates may be divided into seven classes. 



Class Cyclostomata 



Cyclostomes are the round-mouthed lamprey eels and hag-fishes. 

 They have a persistent notochord, lack a biting jaw, and the beginnings 



