476 ELEMENTARY ANATOMY. [LESS. 



as is the case in the Lancelet. This, however, is a solitary 

 exception. In all other cases the water is mechanically 

 propelled by muscular contractions, and so made to pass over 

 delicate filaments or sheets (lamellae) of membrane attached 

 to a greater or less number of the visceral arches. 



We may have (as in Myxine and in Bdellostoma] the gills 

 in the form of a folded lining to six or seven pouches placed 

 in a linear series on each side of the body, each pouch com- 

 municating with the interior of the oesophagus by an aperture, 

 and thence receiving water. Each sac may open on the 

 surface of the body by a separate aperture (as in Bdellostoma 

 and in the Lamprey), or each sac may open into a tube 

 running along each side of the body external to the sacs, and 

 terminating by an aperture placed far back on the ventral 

 surface of the body (as in My.viue}. In the latter case the 

 water coming from the sacs passes out at one aperture on each 

 side of the body. 



The sacs may open internally, not into the oesophagus but 

 into a median canal placed ventrally to the oesophagus (as in 

 the Lamprey). This canal ends blindly at its post-axial end, 

 but pre-axially opens into the mouth by a valvular aperture. 



These gill-sacs are not supported by solid visceral 

 (branchial) arches internally, but may be, as in the Lamprey, 

 protected externally by a cartilaginous framework or basket, 



FIG. 401. SKELETON OF HEAD AND GILLS OF LAMPREY. 



hinder part of the external (paraxial) cartilaginous skeleton of the gills ; an t 

 auditory capsule ; A, hyoid ; ;/, neural arches ; /, palato-quadrate arch, 



which, as we saw in Lesson VI., is a paraxial, skeletal 

 structure. 



The several gill-sacs are separated from each other by 

 partitions which are not solid, cartilaginous structures. 



The gills may be supported internally by solid (osseous or 

 cartilaginous) " branchial arches," which, as we have seen in 

 treating of the skeleton, are those visceral arches which suc- 

 ceed the hyoidean and mandibular arches post-axially. 



