10 MORPHOLOGY OF THE VERTEBRATA. 



HEART 



Is a muscular sac invested in pericardium, situated just in 

 front of pectoral arch, below the, hind part of the gills. It 

 consists of an auricle, which receives venous blood from a sinus 

 venosus, and a ventricle, which is continued into an aortic bulb, 

 which, in virtue of its striped muscular fibres, is rhythmically- 

 contractile. The interior of the bulb is provided with four 

 transverse rows of valves. Sinus venosus — a development of the- 

 termination of the venous system, is situated just behind auricle, 

 and is only partly covered by the pericardium. The wall of the 

 auricle is thin, its only function being to impel blood into the 

 ventricle. The ventricular wall having a more powerful function to 

 perform, viz., to impel the blood throughout the entire body, is in 

 consequence proportionally thick. It is internally provided with 

 stout muscular bands — the musculi papillares — which project into- 

 its cavity. There are two membraneous valves at the auriculo- 

 ventricular opening, which prevent, after the contraction of the 

 auricle, the reflux of blood into that chamber. 



The pericardium is perforated, communicating with the cavity , 

 of the peritoneum by the pericardio-peritoneal canal, which 

 bifurcates, after leaving the pericardium, into two canals, which, 

 after diverging from one another, open into the peritoneum 

 opposite the termination of oesophagus. The course of the 

 circulation is as follows : — The blood is collected by the great veins 

 into the sinus venosus, which pours its contents into the auricle, 

 by the contraction of which the ventricle becomes filled. The 

 ventricle, contracting in its turn, propels the blood through the 

 aortic bulb to the gills, where it is aerated. From the gills it is 

 carried by efferent branchial vessels, which by their confluence form . 

 the dorsal aorta — from where the blood is distributed to all parts of 

 the body. From this it is observed that the aerated blood which 

 has passed through the gills is not returned to the heart, but is 

 driven from the branchiae into the aorta, from where it is conveyed 

 all over the body. The propulsive force necessary for this is- 

 derived from the heart itself, the valves in the aortic bulb giving , 

 full effect to the contraction of the muscular wall of the bulb, 



RESPIRATORY ORGANS 



Are in the form of five fixed flattened pouches. The branchial 

 chamber is divided by four partitions into five chambers, and the 

 branchial laminae are attached by the whole of one margin to each 

 septa. In this manner five pouches are formed, each of which opens 

 inwardly into the pharynx by a separate slit. The branchial 

 clefts are not covered^^y li pp|eKailaK«pembrane as in the cod. « 



