332 CHORD AT A. 



papilla called the barbel. Above the mouth and quite free from it are 

 two small openings on each side. These are the nares. 

 Respiratory, each nasal sac having an anterior and a posterior nas 

 opening directly to the exterior. There is no external 

 opening of the ear. At the hind-end of the head there is on each side a 

 movable plate formed of several bones, called the operculiuii. If this 

 be raised it exposes the four pairs oi gills ^ consisting of long rows of gill- 

 filaments, with large clefts between them, leading into the pharynx. In 

 front of the gills on the first cleft is a vestigial gill, the pseudobranch. 

 The gills of the haddock appear very different from those of the skate, 

 but they are developed in a similar manner. In the skate the clefts are 

 narrow, the filaments short and the body-wall between the clefts broad. 

 In the haddock the clefts are wide, the filaments long and the inter- 

 mediate body-wall reduced to a minimum. In addition the gills are 

 covered over by an operculum. 



The skate takes water in at the spiracle and passes it out by the gill- 

 clefts, but the haddock normally takes water in at the mouth and passes 

 it out through the gill-clefts, the operculum being opened and shut by 

 special muscles. 



Just behind the operculum and situated laterally are the large 

 pectoral fills. Ventrally and slightly forwards are the paired pelvic fins. 

 In many Teleostomi the pelvic fins are far back, as in the skate, but in the 

 GadidcB they are oh^n jugular (on the neck) in position, moving forwards 

 during development. The larval haddock has, in addition to these fins, 

 a continuous median fin stretching along the dorsal surface round the 

 tail and forwards to the anus on the ventral side {cf. Myxine). In later 

 life this fin breaks up into three dorsals, a caudal and two anah\ by 

 differential growth and atrophy of the intermediate parts. The tail-fin 

 is symmetrical, the dorsal and ventral halves being equal, but the end of 

 body bends up into the dorsal half, hence the tail is homocercal (see Pisces, 

 p. 435)' All the fins have the same structure, consisting of a delicate 

 double fold of membrane supported on a series of elastic skeletal dermal 

 fin-rays. Just in front of the first anal fin is a small cloacal depression 

 into which open three apertures. The anterior is the amis, the inter- 

 mediate the genital aperture and the posterior the urinary aperture. 

 If the skin be carefully dissected off one side there can be noticed fine 

 superficial nerves supplying the lateral line and the fins. They arise 

 mainly from the Vth and Xth cranial nerves. Below these the whole 

 lateral wall of the body is formed of diagonal myomere 

 Muscular, muscles, separated by connective-tissue myocommata {cf. 

 Affiphioxus). From a little way behind the anus the rest 

 of the body backwards, usually known as the tail, is composed almost 

 entirely of these myomere muscles. Their alternate contractions serve 

 to move the "tail" and caudal fin and thus propel the body. This 

 method of locomotion is similar to that of Aniphioxus and is also found 

 in many Elasmobranchii : the skate itself has adopted a different method 

 of progression by the pectoral fins, which in the haddock merely act as 

 balancing, steering and stopping organs. 



The perivisceral cavity may now be opened up by a median ventral 

 incision from chin to anus. The cavity is completely divided into two 

 parts, the anterior pericardial cavity and the posterior abdominal cavity. 



