STRUCTURE OF HADDOCK 623 



into the rectum, which has an aperture apart from those 

 of the genital and urinary ducts. There is no spiral valve, 

 and there are no abdominal pores. A pancreas is absent ; 

 perhaps the pyloric caeca take its place. (In some Teleo- 

 steans the pancreas, apparently absent, is combined with 

 the liver.) The peritoneum is darkly pigmented. 



Respiratory system. — Water that passes in by the mouth 

 may pass out by the gill-clefts ; the branchial chamber is 

 also washed by water which passes both in and out under 

 the operculum. The gill-filaments borne on the four 

 anterior branchial arches are long triangular processes, 

 whose free ends form a double row. As there are no 

 partitions between the five gill-clefts, the filaments pro- 

 ject freely into the cavity covered by the operculum. On 

 the internal surface of the operculum lies a red patch, 

 the pseudobranch or rudimentary hyoidean gill. Inspira- 

 tion and taking food into the mouth are associated with 

 the retraction of the hyoid apparatus ; expiration and 

 swallowing are associated with the protraction of the 

 hyoid arch. The usual retractor of the lower jaw is absent 

 in Teleosts, and the lowering of the lower jaw comes about 

 automatically in the retraction of the hyoid arch and the 

 raising of the operculum — in short, in the inspiratory 

 phase. A large and quaint parasitic copepod — Lerncea 

 branchialis — is often found with its head deeply buried 

 in the tissues of the gills and head. Many related forms 

 are common on fishes. 



The swim-bladder lies along the dorsal wall of the 

 abdomen ; the duct which originally connected it with 

 the gut has been closed. The dorsal wall of the bladder 

 is so thin that the kidneys and vertebrae are seen through 

 it ; the ventral wall is thick, and bears anteriorly a large 

 vascular network or rete mtrabile, which receives blood from 

 the mesenteric artery and returns blood to the portal vein. 



Circulatory system. — The heart lies within a pericardial 

 chamber, separated by a partition from the abdominal 

 cavity. The blood from the body and liver enters the 

 heart by the sinus venosus, passes into the thin-walled 

 auricle, and thence to the muscular ventricle. From the 

 ventricle it is driven up the ventral aorta, the base of 

 which forms a white non-contractile bulbus arteriosus. 



