448 SUB-PHYLUM CEPHALOCHORDA. 



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 cycle is completed by the capillaries which form the 

 hepatic vein. 



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, and also opening into the body cavity by a variable number of 

 funnels, most numerous in the nephridia lying in the middle of the 

 pharynx. Each funnel ends in a blind knobbed cell or solenocyte (like 

 that on the nephridium of some Polychsetes) from which a long flagellum 

 projects into the tube of the funnel. The vessels of the primary gill- 

 bars and of the tongue-bars form an anastomosing 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. Their develop- 

 ment is unknown. 



* 



Reproductive system. --The sexes are separateand 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. 216, G.). Each lies in a "genital chamber" 

 formed in development by constriction from the cavity of 

 the lower part of the primitive segment. 



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 3-!-^ in. in 

 diameter. The segmentation is complete and almost equal 

 (Fig. 221). 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 equatorial, 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. The blasto- 

 sphere, which is the final result of segmentation, invaginates 

 to form a gastrula. 



Along the mid-dorsal line of the gastrula the ectoderm 

 cells sink in slightly so as to form a groove. This is the 



