THE ANIMAL KINGDOM 



21 



The body is covered by a thin external cuticula secreted by the simple 

 epithelial epidermis. Beneath the skin and visible through it, are sixty 

 pairs of myotomes which alternate with one another along the two sides 

 of the body. As in the vertebrates generally, these myotomes are greatly 

 thickened along the dorsal side of the body. Each myotome is V-shaped 

 with the apex of the V pointed forward. 



The mouth, surrounded by a circle of tentacles, leads directly into 

 the elongated pharynx, the walls of which are perforated by numerous 

 gill-slits. A ciliated groove, which is similar in function and in relations 

 with the endostyle of urochords, extends the entire length of the pharynx. 

 Opposite it, in the roof of the pharynx, is a somewhat similar epipharyn- 

 geal groove. The liver is a hollow tubular sac, which opens into the 



CIRRI 

 ORAUHOOOI 



Fig. 14. — Amphioxus, in ventral and side views. Metamerism, lacking in uro- 

 chordates, and scarcely evident in hemichordates, is strikingly shown by amphioxus. 

 Whether this metamerism is inherited from annelid-like ancestors or is a convergent 

 trait independently acquired, is a moot question in morphology. (Redrawn after 

 Kirkaldy.) 



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, 

 Umited 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. 331) 



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. 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 



