THE MAIN FACTS CONCERNING THE LANCELET. 29 



Immediately above the alimentary canal is a tough elastic rod, 

 the Notochord (nt\ reaching from end to end of the body. Above 

 this again, lies the Spinal Cord, a cylinder having slightly less 

 diameter, marked along the under side by black pigment cells. 

 No brain is apparent, but the more anterior part is slightly thicker 

 than posteriorly. Branching nerves are given off at intervals to 

 the various organs. 



Sense Organs are rudimentary ; a pigmented patch at anterior 

 end of the spinal cord, serves as " eye," while a ciliated pit at the 

 same point may be olfactory in function. 



The Muscular System is well developed and enwraps the various 

 organs as in a sheath. Its elements are composed of striated fibres, 

 arranged principally in peculiar < shaped bundles or myotomes 

 numbering 61 62 on either side of the body. On the ventral 

 surface are subsidiary muscles running transversely, whose function 

 is, by contraction, to expel water from the atrial cavity. 



Blood System. No definite heart is present. Impure blood 

 from all parts of the body is collected into a ventral vessel, the 

 cardiac aorta, which by the contractility of its walls is able to propel 

 this blood into paired vessels, aortic arches, that pass along each 

 alternate (primary) arch. The blood, purified in these vessels by 

 intimate exposure to the water passing through the gill slits, is 

 emptied into a right or left dorsal aorta according as it comes from 

 right or left side of the pharynx. The right and left dorsal aortse 

 unite to form, at the hinder end of the pharynx, a single vessel which 

 passes backwards on the dorsal side of the intestine. From the 

 dorsal aortaa the purified blood is distributed to the organs of the body. 



Reproductive Organs. The sexes are separate ovaries and 

 testes have the same outward appearance, that of two rows of 

 enlargements along the body-wall, external to the atrial cavity and 

 parallel with the pharynx, a row on either side of the body. The 

 products escape by rupture of the atrial wall, into the atrial cavity 

 and escape by the atrial pore, or it may be by the mouth, by passing 

 through the gill clefts. 



Want on Internal Symmetry. It is a remarkable fact about 

 Amphioxus, that the internal organs do not show bilateral symmetry, 

 that is, are not arranged in balanced pairs. Thus any particular gill 

 cleft on one side is opposite a gill arch on the other side, instead 

 of opposite another cleft. The aortic arches necessary partake also 

 of this asymmetry. The muscle segments or myotomes, nearly all 

 the nerves arising from the spinal cord, and the reproductive glands 

 are also alternate on the two sides of the body. The liver too is 

 asymmetric, being turned to the right side, while the anus opens 

 on the left side. 



