468 SUB-PHVYLUM CEPHALOCHORDA. 
pharynx to form the right and left dorsal aorte, which join at the 
hinder end of the pharynx to form a single vessel running backward 
over the intestine, and breaking up into capillaries on its wall. From 
the right dorsal aorta there arises a complex of vessels supplying the 
anterior region. From the capillaries of the intestine the blood is 
collected in a sub-intestinal vein, which again breaks up in the 
cecum. The circle is completed by the capillaries which form the 
hepatic vein. The course of the circulation is essentially that of a 
Vertebrate. ° 
Excretory system.—Boveri has described an elaborate system of 
about ninety pairs of #ephridia lying in the dorso-lateral wall of the 
pharynx. ‘They are short tubules, with a single opening into the 
atrial cavity.’ On the inner aspect there are a number of blind funnels 
projecting into the body cavity. On these funnels are set a number 
of solenocytes (like those on the nephridium of some Polycheetes), 
which are long tubular cells (Fig. 251, ¢.), closed above by a knob con- 
taining the nucleus, from which hangs down a long flagellum. The 
vessels of the primary gill-bars and of the tongue-bars form an anasto- 
mosing vascular plexus, called a glomerulus,'over the tubules. In 
number the tubules correspond to the primary gill-clefts, and are 
therefore in origin segmental structures. They are regarded by their 
discoverer as equivalent to the pronephric tubules of Vertebrates. 
They develop from the mesoderm. 
Reproductive system.—The sexes are separate and similar. 
The organs are very simple, and without ducts. They form 
twenty-six pairs of horseshoe-shaped sacs, lying along the 
inner wall of the atrial cavity in segments ten to thirty-five 
on each side (Fig. 248, G.). Each lies in a “genital 
chamber” formed in development by constriction from the 
cavity of the myotome. 
In the mature female the ovaries are large and con- 
spicuous ; the oya burst_into the atrial. cavity, whence they 
pass out by the atriopore. 
The testes are like the ovaries; the spermatozoa burst 
into the atrial cavity, and pass out by the atriopore. The 
eggs are fertilised in the surrounding water. 
Development.—The fertilised ovum is about 4% in. in 
diameter. ‘The segmentation is complete and almost equal 
(Fig. 253). The first cleavage is vertical, and divides the 
ovum into two equal parts; the second is also vertical, 
along a meridional plane at right angles to the first, and 
the result is four equal cells. The third cleavage is equa- 
torial, and gives rise to four larger cells (or macromeres) 
below or towards the vegetative pole, and to four smaller 
cells (or micromeres) above or towards the animal pole. 
