104 



SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA). 



which passes back through the pericardial cavity to become 

 continuous by a valvular aperture with the straight intestine, 

 which opens at the anus. The anterior end of tlie intestine is 

 slightly dilated, and receives the opening of the bile-duct, and 

 in Petromyzon the intestine is provided with a longitudinal fold 

 or valve, which takes a slightly spiral course, 



In Myxinoids the naso-palatine canal (pituitary pouch) opens into the 

 hinder part of the buccal cavity. This may be taken to mark the junction 

 of mouth and pharynx. The opening is guarded by an epiglottis-like 

 valve and directly behind there is a velar membrane of a peculiar form 

 hanging from the dorsal wall and supported by the so-called pharyngo- 

 branchial cartilages (Fig. 53, 2, 3). The part of the alimentary canal 

 immediately succeeding the mouth and receiving the internal gill aper- 

 tures should be called pharynx, though it is 

 commonly termed oesophagus. In Petromyzon 

 a velar fold marks the junction of thesuboeso- 

 phageal tube or bronchus with the hinder part 

 of the mouth. 



There is a gall bladder, but in the 

 adult Petromyzon the bile-ducts and gall- 

 bladder atrophy and the liver cells be- 

 come filled with fat (Schneider). A 

 pancreas and spleen appear to be absent, 

 and the mesentery is very imperfectly 

 developed. The anus is placed in the 

 median ventral line in a shallow pit im- 

 mediately in front of the urogenital 



Fig. 54. — Head of Petromyzon oDPninO' 

 marinus, seen from below "P^ &• 



?eethofthe\uifcaf?uJ:r" The respiratory _ Organs consist of a 



number of branchial sacs, in which the 

 branchial lamellae are contained. In Petromyzon there are 

 seven pairs of these sacs, and each of them opens independ- 

 ently to the exterior in the anterior region of the body, by 

 a short external branchial passage (Fig. 48), but internally 

 they open into a longitudinal suboesophageal tube (bronchus), 

 which opens into the buccal cavity in front (Fig. 51), and ends 

 blindly behind. The anterior opening of the suboesophageal 

 tube is guarded by a membranous velar fold. 



In Myxine there are six pairs of branchial sacs (seven have 

 been observed in rare cases). In Bdellostoma the number is more 

 variable ; seven pairs appear to be the usual number, but 



