THE CHORDATES 



387 



substrate to which it attaches by its anterior adhesive glands. It loses its 

 tail and is transformed into an adult. Notochord and nerve cord are 

 resorbed, only the ganglion and neural gland remaining as traces of 

 the latter. 



176. Subphylum Cephalochordata 



Amphioxus and a related genus of small, superficially fish-shaped 

 chordates constitute the subphylum Cephalochordata. Species occur in 

 the United States in coastal waters south from Chesapeake {Amphioxus 

 virginiae) and Monterey {A. calijorniense*) Bays. They usually lie buried 

 in sand with only their anterior end protruding, but they can also swim 

 fairly well. 



The body of Amphioxus (Fig. 20.1, C) is elongate, tapers at each 

 end, and is compressed from side to side. A dorsal, a caudal and a 

 ventral fin lie in the median plane of the body, and a pair of long 

 finlike metapleural folds are present ventro-laterally. Dorsal and ventral 

 fins are supported by blocks of connective tissue, but these fins and 

 folds are apparently not large or strong enough to keep the animal on 



Figure 20.3. A diagrammatic lateral view of Amphioxus. White arrows represent 

 the course of the current of water; black arrows that of the food. 



an even keel, for Aynphioxus spirals as it swims. Swimming is accom- 

 plished by the contraction of longitudinal muscle fibers in the body 

 wall that are arranged in segmental, <-shaped muscle blocks, or myo- 

 meres. These can easily be seen through the thin skin. Successive myo- 

 meres are separated by connective tissue septa to which the muscle 

 fibers are attached. Shortening of the body is prevented by an unusually 

 long notochord (Fig. 20.3) that extends farther anteriorly than in any 

 other chordate, an attribute after which the subphylum is named. 



Water and minute food particles are taken in through the oral 

 hood, whose edges bear a series of delicate projections, the cirri, that 

 act as a strainer to exclude larger particles. The inside of the oral hood 

 is lined with bands of cilia called the wheel organ, which, together with 

 cilia in the pharynx, produce a current of water that enters the mouth. 

 The mouth proper lies deep within the oral hood and is surrounded by 

 twelve velar tentacles. 



* Branchiostoma Costa, 1834 has priority over Amphioxus Yarrell, 1836 as the generic 

 name for these animals, but there is some question as to the adequacy of Costa's descrip- 

 tion and hence as to the validity of his name. 



