CYCLOSTOMATA 



vibration of the two bodies, eggs and 

 sperm are extruded in close contact. 

 Tlie eggs sinlv to tlie gravelly bottom 

 in a place that has been cleaned up for 

 a "nest." The nest has been prepared 

 by moving many stones, Ijoth males 

 and females using the buccal funnel for 

 this purpose. 



The egg, which measures about a 

 millimeter in diameter, goes rapidly 

 through cleavage, blastula, and gas- 

 trula stages and forms a tiny larva 

 which has come to be known as "Aui- 

 moccetes," because when first discovered 

 it was believed to represent a separate 

 genus of lowly chordates. 



The most significant characteristics 

 r)f the Ammoccctes larva (Fig. 51) are 

 those in which it strikingly resemljles 

 Amphioxus: (1) a hood-like upper lip 

 resembling the oral hood of Amphioxus ; 

 (2) a well-defined parietal or median 

 eye ; (3) a food-concentrating apparatus 

 consisting of an endostyle and dorsal 

 mucous groove; this implies a similar 

 mode of feeding; (4) median fins con- 

 tinuous and unspecialized. 



Ammoccetes is more advanced than 

 Amphioxus in other respects, as for 

 example: — the paired eyes that lie deep 

 in the head, a small cranium, a much 

 more advanced brain, a reduced num- 

 ber of branchial clefts, a concentrated 

 kidney (pronephros), a distinct ventral 

 heart, a liver with a gall bladder. 



Fig. 51.- 



larva 



AmmoocBtes 

 of Pelromyzon, enlarged sagittal 

 section, bd, bile duct; eg, ciliated 

 groove; da, dorsal aorta; fv, cav- 

 ity of brain; g, gills; i, intestine; 

 k, kidney (mesonephros); I, lip; 

 li, Liver; nl, nasal aperture; nl, 

 notochord; n, neural tube; oe, 

 OBSophagus; p, oral papillaj; pe, pericardium with heart removed; pn, pineal 

 eye; pr, pronephros showing nephric funnels; sp, spiral valve; th, thyroid forming 

 from endostyle; v, velum; va, ventral aorta; ve, intestinal vein. (From Lan- 

 kester's "Treatise on Zoology," Vol. IX, Goodrich.) 



