CYCLOSTOMATA 



vibration of the two bodies, eggs and 

 sperm are extruded in close contact. 

 The eggs sink to the gravelly bottom 

 in a place that has been cleaned up for >^s> 

 a "nest." The nest has been prepared ' 

 by moving many stones, both 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- 

 trata stages and forms a tiny larva 

 /which has come to be known as " Am- 

 mocoetes," because when first discovered 

 it was believed to represent a separate 

 genus of lowly chordates. 



The most significant characteristics 

 of the Ammoccetes larva (Fig. 51) are 5 

 thtfse in which it strikingly resembles ^ 

 /Aniphioxus: (1) a hood-like upper lip 

 resembling the oral hood ofAniphioxtis ; 

 (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 eves that lie dee 

 in the head, a. smp.11 nrajnjmm, a muc 



1. 



ft ,'- 

 *' 



"& 



FIG. 5!. Ammocoetes larva 

 of Petromyzon, enlarged sagittal 



section, bd, bile duct; e#, ciliated 



more advanced brain, a reduced num- 



E 1 _ ot/v-'iAiA-'i.A. i/u/j (LPJ.J.C; VALIV./LJ. oj/. v^inct IA>VJ. 



_er_pt branchial^clefts, a concentrated groove; da, dorsal aorta ;/, cav- 



kidriey (pronephros) /adistinct ventral % of brain J* gills;-*, intestine; 



notochord; n^ neural tube; oe-, 



oesophagus; ^ oral papillae; pf, pericardium with heart removed; p, pineal 

 eye; p^ pronephros showing nephric funnels; s^, spiral valve; ifc, thyroid forming 

 fr.)m endostyle; ^ velum; no, ventral aorta; , intestinal vein. (From Lan- 

 J *jster's " Treatise on Zoology/' Vol. IX, Goodrich.) 



