RESPIRATION. 105 



there may be six pairs, or seven on one side and six on the 

 other (heterotrema), or there may be more than seven (up to 

 fourteen pairs). In both genera the sacs are connected directly 

 with the oesophagus (pharynx) by internal branchial tubes, and 

 with the exterior by external branchial tubes, but whereas in 

 Bdellostoma each of the external branchial passages opens separ- 

 ately by a small aperture (Fig. 61), in Myxine all the external 

 branchial tubes of the same side are directed backwards, and 

 unite together before opening to the exterior by a common 

 opening at the hind end of the branchial region. In both genera 

 there is on the left side a tube, the oesophageo-cutaneous duct, 

 which leads directly from the oesophagus behind the last gill 

 sac, to open with the external branchial tube of the last left 

 gill sac in Bdellostoma, and with the left gill aperture of 

 Myxine. The oesophageo-cutaneous duct is much wider than 

 the external branchial tubes. 



In all Marsipobranchs respiration can be effected while the 

 animal is adhering to foreign objects by the suctorial mouth. 

 In Petrom'yzon respiration is effected by taking in water through 

 the external branchial openings into the branchial sacs, and 

 then expelling it again by the same way. In Myxinoids the 

 water is said to enter through the nasal tube, which communi- 

 cates with the mouth through the posterior nasal passage, and 

 passes out by the branchial sacs ; but the nasal passage is a 

 narrow one, and perhaps hardly sufficient to supply all the 

 respiratory water. Moreover, it would not be available when 

 the animal's head is buried in the tissues of its prey. It would 

 appear more probable that some at least of the inspiratory water 

 enters through the oesphageo- cutaneous duct. 



In Petromyzon the branchial basket plays an important part 

 in respiration. In expiration it is compressed by the trans- 

 verse muscles ; in inspiration it recovers by its own elasticity. 

 In Myxinidae it is possible that the huge lingual apparatus may 

 play some part in bringing about inspiratory dilatation of the 

 pharynx, but it has been asserted that the inflow of water in 

 these animals is effected through the nasal canal by ciliary 

 action. 



In the respiration of Ammocoetes water is taken in through the 

 mouth and passed out by the clefts. The expulsion of the water 

 is effected by muscular constriction of the branchial region ; the 



