42Q CEPHALOCHORDA. 



(a) On the floor of the atrial chamber clusters oi cells occur which 

 form the so-called renal papilla?. These have been experimentally shown 

 to possess the power of taking up foreign substances introduced into the 

 body. 



(b) Professor. E. Ray Lankester discovered a pair of short pigmented 

 funnels in the region of the twenty-seventh myotome, which open 

 into the atrial cavity, and perhaps communicate with the dorsal ccelomic 

 space. They hardly seem to be nephridia, and their relations are 

 doubtful. 



(c) More recently 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. 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 

 development is unknown. 



(d) Hatschek discovered in the anterior region of the larva a 

 nephridial tube which is absent in the full-grown adult. Its significance 

 is very doubtful, but it perhaps represents the connection between the 

 left of the pair of collar pouches and the gut. 



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. 177, 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 ^^ in. in 

 diameter. The segmentation is complete and almost equal 

 (Fig. 182). 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 



