AMPHIOXUS. 63 



and the branchial pore. The young are less at home in the sand ; probably they do 

 not take to it until nearly adult. 



When full grown, the body is of a light flesh color, slender, compressed, and taper- 

 ing to a point at head or tail. Mouth and vent are a little toward the left side, and 

 each is a little way from the extremity. The former is an elongate oval, surrounded 

 by about twenty-five tentacles ; in front of it, around the beak formed by the end of the 

 notochord, along the back, and around the tail from the branchial pore backward, there 

 is a sort of fin or dermal expansion. A third of the length from the end of the tail, in 

 front of the vent, there is an opening or pore into the branchial chamber (atrium or 

 branchium) which allows the escape of the water entering through the gill openings. 

 This chamber lies below the pharynx ; when full, it is rounded and transparent ; when 

 collapsed it is marked by a translucent fold of its walls along each flank. 



On the flanks the muscles bear some resemblance to those of the fishes. Those of 

 the two sides are brought close together toward the ' fin rays,' but are separated over the 

 notochord and viscera. They .appear to be arranged in transverse series (about sixty), 

 each band forming an angle directed forward. They are formed of series of more or 

 less regular and overlapping plates, and do not extend quite to the end§ of the verte- 

 bral column. Besides these, there are transverse and longitudinal abdominal muscles, 

 others about the gills and tentacles, and others around the mouth, vent, pore, and 

 pharyngeal ring. 



The alimentary canal is very simple, nearly straight, lined with vibratile cilia, and 

 consists of the mouth, pharynx, oesophagus, stomach, and intestine. A constriction 

 caused by a ring of cartilage called the j^haryngeal ring separates the mouth from the 

 ]|harynx. The latter extends to about the middle of the total length; from it the nu- 



terous gill-openings, varying in number with age, let the water pass into the branchial 

 lamber. The oesophagus is short ; in it the food that has been strained from the 

 ^yater is compacted into a mass or string, from which the nourishment is ground or 

 shueezed in the stomach. The intestine begins above the pore, at the beginning of the 

 lower fin, and continues to a point a little in front of the end, where the vent opens 

 downward at the side of the fin. 



Such a skeleton as that of JBraiicJdostoma is hardly worthy the name. What 

 there is of it is composed of the softest of cartilage. There is nothing whatever 

 resembling a skull. A long slender rod, the notochord, pointed at each end, forms the 

 vertebral axis. Outside the rod is a membranous envelope ; inside it is a series of 

 thin, flat discs placed side by side. At each side of this axis are the body muscles ; 

 below it are the viscera ; and above it the neural cord separates it from the fin rays. 

 The pectoral skeleton is united with the dorsal only by ligament. The base of the 

 .former is somewhat like a sternum. It supports a ring of cartilages surrounding the 

 mouth, the pharyngeal ring, and the branchial arches which rise like ribs toward 

 the back. Each of the segments composing the mouth ring bears on its front edge a 

 slender rod, the support of a tentacle. 



The spinal cord is slender and tapering; it extends from within a short distance of 

 the front end of the notochord to its posterior extremity. Antei-iorly, for a little way, 

 it is slightly larger ; on its left side there is a slight prominence toward a ciliated pit, 

 supposed to be a nasal fosse. Posteriorly, just above the end of the dorsal cord, there 

 is a short upward bend, ending in a knob. This neural or spinal cord is a mere string, 

 lying upon the notochord, enclosing a narrow canal. Anteriorly the canal is closed, 

 and near the end in front there is a spot of pigment, said by some to be a rudimentary 



