I1l4 CEPHALOCHORDATA CHAP. 
connected with it a simple hepatic portal blood system. There 
is a respiratory circulation, the contractile ventral vessel which 
represents the heart sending the colourless blood forward to the 
respiratory pharynx to be purified. The body-wall is segmented 
into over fifty myotomes. There are numerous separate nephridia 
which develop from the mesoderm and open into the atrium. 
The brain remains undeveloped, being scarcely distinct from the 
spinal cord. There are two pairs of cerebral nerves, and many 
spinal, in which the dorsal and ventral roots or nerves do not 
unite. The sense-organs are simple; there are no paired eyes 
and no auditory organs. The sexes are separate; the gonads are 
metamerically arranged on the body-wall, and have no ducts: 
they burst into the atrium. In the development the segmenta- 
tion is complete, a gastrula is formed by invagination, the 
nervous system is formed from the dorsal epiblast, the noto- 
chord from the hypoblast, and the mesoderm arises from meta- 
meric coelomic pouches. The body-cavity is an enterocoele. 
The gill-slits are at first perforations of the body-wall opening 
from the pharynx to the exterior, which later become enclosed 
by the development of the atrium. 
ANATOMY. 
External Characters.—Amphioxus’ is about 1} to 24 
inches in length, slender, somewhat translucent, and pointed at 
both ends (Fig. 69). It lives in shallow water and burrows in 
the sand, head first, with great rapidity. It frequently remains 
with the anterior end protruding from the sand. When on the 
surface it les on one side. It is said to swim freely at night. 
The head end is rather the thicker, and the anterior two-thirds 
of the ventral surface are flattened (Fig. 70, A), and may be 
slightly ridged longitudinally. The lateral edges of this flat 
area project as metapleural folds (Fig. 70, m#.p/), which begin 
anteriorly at the edges of the external mouth, and die away in 
the middle line posteriorly behind a median opening, the 
atriopore (Fig. 70, atrp). From this point a ventral median 
5 ’ LP) 
fin (vent.f) extends backwards around the pointed posterior end 
1 Although the correct systematic name of the commonest species is Branchio- 
stoma lanceolatum (Pallas), it is convenient in non-systematie usage to employ 
the term ‘‘ Amphioxus,” which is in general use in zoological laboratories. 
Sty ay 
