448 CYCLOSTOMATA [CH. 



The pericardium is not completely separated from the remainder 

 of the body-cavity, and the genital organs take the form in both 

 sexes of a single median ridge projecting into the body-cavity (21, Fig. 

 219). No connection of the testis tubules with the kidney tubules 

 exists, nor is there any trace of an oviduct, since both ova and 

 spermatozoa are freely shed into the body-cavity and escape by two 

 abdominal pores or simple openings in the body-wall placed 

 ventrally to the openings of the kidneys. Inasmuch as these latter 

 open directly to the exterior and are quite independent of the 

 opening of the intestine, which is placed more ventrally, we may 

 state that no cloaca has yet been developed. 



Living Cyclostomata, represented by a single class which may 

 be called Marsipobranchii (Gr. uapo-tTro?, a pouch), 



Classification. .. . .. . 



are divided into two orders : (i) PETROMYZONTIDA, 

 (ii) MYXINOIDEA. 



(i) In the first order, familiarly known as the Lampreys, the 

 pituitary body appears as a Wind process from the nasal sac : each 

 gill-sac opens directly to the exterior, jind the hyperpharyngeal 

 groove is separated from the rest of the alimentary canal as a 

 distinct tube, the so-called oesophagus. 



The Lampreys (Petromyzoii) are conspicuous in the early spring, 

 when they ascend small brooks to spawn. Several species inhabit 

 the rivers of Great Britain, Canada and the United States, but the 

 differences between them are trifling, depending mainly on the 

 development of the horny teeth covering the tongue. One species, 

 Petromyzon marinus, attaining a much larger size than the others, 

 inhabits the sea. It may reach a length of three feet, whereas the 

 other forms do not grow longer than from ten to twelve inches. 

 The eggs of Lampreys develop into a most interesting larval form 

 which stands in many respects nearer to the other Craniates 

 than does the adult and supplies an intermediate stage between 

 Amphioxus and an ordinary Craniate. This larva is called the 

 Ammocoetes, and it's mode of life resembles on the whole that of 

 Amphioxus. Like that animal the Ammocoetes lives on what is 

 brought by the currents of water, produced by the cilia inside the 

 velum. The thyroid gland, which, as we have seen, represents the 

 endostyle, remains open, and still performs its primitive function of 

 secreting a cord of mucus, which is carried up dorsally by a ciliated 

 groove, the peripharyngeal band, situated just behind the velum. 

 The hyperpharyngeal groove is represented by a dorsal strip of 

 ciliated cells, the current produced by which sweeps the mucus 





