CVCLOSTOMATA 429 



tinuous. A gall-bladder is present, and also a bile duct open- 

 ing into the gut. In its mode of life, and especially in the 

 manner in which it obtains its food, Ammocoetes presents a 

 most remarkable resemblance to Ampliioxus and the Ascidians. 

 In the median line of the pharyngeal floor there is an open 

 groove, the hypopharyngeal groove or endostyle, and a tract of 

 ciliated cells along the dorsal wall represents a hyperpharyngeal 

 groove : connecting the two in front there is a peripharyngeal 

 cilated groove.-^ The Ammocoetes feeds on small food particles 

 carried through the mouth into the pharynx by currents of 

 water produced by ciliary action. The food becomes entangled 

 in strings of mucus probably secreted by the cells lining the 

 endostylar groove. The mucus is then swept upwards in the 

 pharyngeal groove, and finally wafted backwards to the stomach 

 and intestine by the cilia of the hyperpharyngeal band. The 

 skin exhibits the remarkable peculiarity of containing a peptic 

 ferment capable of digesting proteids in a '2 per cent solution 

 of hydrochloric acid. As the larva lives buried in the mud, the 

 epidermic secretion probably helps to keep the skin free from 

 bacteria, microscopic spores, and fungoid, or other parasitic 

 growths.^ The young Lamprey lives as an Ammocoetes from 3-4 

 years, and then in the course of a few weeks in the winter it ixnder- 

 goes a metamorphosis, losing its larval characters and acquiring the 

 structure and habits of the adult. During this period the buccal 

 funnel is completed and teeth are developed. The eyes approach 

 the surface and become functional. The continuity of the median 

 fins becomes interrupted. The endostylar groove becomes trans- 

 formed into a thyroid gland, the gall-bladder disappears, and the 

 bile duet becomes obliterated and changed into a mass of small 

 folKcles. The skull and branchial basket complete their develop- 

 ment. At the same time the pharynx loses its connection with the 

 rest of the alimentary canal and remains as the branchial canal. 

 The so-called oesophagus of the adult is apparently a new formation 

 which grows forwards and acquires a connection with the mouth. 

 It is probable that it represents a hyperpharyngeal groove con- 

 stricted off from the dorsal wall of the pharynx. 



Both the marine Lamprey and the " Lampern " are captured 



' Dohm, Mitth. Zool. Stat. Neapel, vi. 1886, p. 59 ; Shipley, Quart. Journ. 

 Microsc. Sci. xxvii. 1887, p. 325. 



^ R. Alcook, Journ. Anat. and Phys. xiii. (N.S.), 1899, p. 623. 



