468 SUB-PHYLUM CEPHALOCHORDA. 



pharynx to form the right and left dorsal aortse, 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 

 caecum. 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 nephridia 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 Polychaetes), 

 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 ova 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 ^itf m * n 

 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. 



