332 CHORDATA. 
papilla called the da7éel. Above the mouth and quite free from it are 
two small openings on each side. These are the wares, 
Respiratory. each nasal sac having an amtertor 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 operculum. If this 
be raised it exposes the four pairs of ¢¢//s, 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 ina 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 fins. Ventrally and slightly forwards are the paired Ze/vic fins. 
In many Ze/eostomi the pelvic fins are far back, as in the skate, but in the 
Gadide they are often jugu/ar (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 azzs on the ventral side (cf A/yxzne). In later 
life this fin breaks up into three dorsals, a caudal and two anals, 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 Aomocercal (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 
Jjin-rays. Just in front of the first anal fin is a small cloacal depression 
into which open three apertures. The anterior is the avws, the inter- 
mediate the genztul aperture and the posterior the urénary 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. 
Amphioxus). From a little way behind the anus the rest 
of the body backwards, usually known as the /az/, 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 4mphioxus and is also found 
in many Zlasmobranchii : 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 pertvisceral cavity may now be opened up by 2 median ventral 
incision from chin to anus. The cavity is completely divided into two 
parts, the anterior fericardial cavity and the' posterior abdominal cavity. 
