526 HISTORY OF THE HUMAN BODY 



the manner of other non-locomotive forms. The larva is, how- 

 ever, actively free-swimming, and the adult often changes its 

 locality and thus retains the power of rapid motion. 



A striking external feature of Amphioxus is the regular seg- 

 mentation of its muscular system, which shows through the 

 transparent skin and is marked by lines formed by the myocom- 

 mata, each bent in the form of a V, the point directed for- 

 wards. The reproductive organs or gonads, also, are often 

 sufficiently developed to appear through the skin as a succession 

 of square or slightly rounded masses, each corresponding to a 

 segment, yet with those of the two sides placed alternately, 

 as is also the case with the myomeres and the other lateral 

 parts. There is a slightly indicated median fin, supported 

 by minute fin rays, and extending along the entire back, 

 around the tail and upon the ventral side considerably past the 

 anus, throwing this latter opening out of the median line, and 

 dislocating it to the left. Anterior to this and at the termina- 

 tion of the fin is a large and conspicuous opening, the atriopore, 

 through which the water that is continually taken in at the 

 mouth is as continually expelled. The chamber from which 

 the atripore leads is termed peribranchial, and appears at first 

 like an internal cavity; a little investigation, however, shows 

 that it is in reality external and develops in the larva from two 

 longitudinal folds that arise along the sides and grow together 

 ventrally. The region of the body which they enclose is 

 perforated laterally by a very large number of obliquely placed 

 gill-slits, communicating with the pharynx, so that the water 

 taken in at the mouth passes through these slits and thus enters 

 the peribranchial chamber, from which it is finally expelled 

 through the atriopore. A similar device is found in frog and 

 toad tadpoles, and the external outlet, here called the " branchi- 

 pore," varies in position in the different genera, but appears 

 quite high up on the left side in the true frogs (Rana). As 

 the peribranchial chamber develops during larval life, the gill- 

 slits of young larvae open directly to the outside, as in true 

 vertebrates, and suggest that, as in the frog tadpole, this 

 chamber has been developed as a special adaptation to the 



