STRUCTURE OF AMPHIOXUS 523 



divided into two cavities. The right becomes the cavity of the snout 

 in the larva, but is almost obliterated in the adult. The left becomes 

 the pre-oral pit. This anterior coelom pouch may correspond to the 

 head ccBlora of Balanoglossus, and to the bilobed head cavity which lies 

 beneath the eyes of fishes, and forms most of the eye muscles. 



Thirdly, there is a pair of pouches, which form the first pair of 

 muscle segments, and are continued out into the atrial folds. These 

 may correspond to the collar coelom of Balanoglossus (MacBride). 



Two brown canals or atrio-coelomic funnels discovered by Sir 

 E. Ray I.ankester open into the dorsal part of the atrium about the 

 level of the junction between pharynx and intestine, while their 

 anterior ends project into the dorso-pharyngeal coelom about the 27th 

 rnyotome. They are probably diverticula of the atrium. 



Circulatory system. — The blood is colourless, with a 

 few amoeboid cells. There is no definite heart, but the 

 branchial artery is rhythmically contractile. 



This branchial artery lies in the portion of the body cavity which is 

 enclosed by the endostyle, and is the anterior continuation of a large 

 hepatic vein from the caecum. From the branchial artery a series of 

 smaller vessels arise, which pass up the primary gill-bars, and also 

 supply the tongue-bars. These unite on the dorsal surface of the 

 pharynx to form the right and left dorsal aortae, 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. — Bove'ri has described an elaborate system of 

 about ninety pairs of nephridia lying in the dorso-lateral wall of the 

 pharvnx. 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 Polycha;tes), 

 which are long tubular cells (Fig. 300, T.), 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 (sec Fig. 299). 



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 



