COMMON FROG. 75 



vena cava superior may be seen between the root of the left lung and the 

 left auricle where it dips down on its way to the sinus venosus. The liver 

 with its right, central, and left lobes lies inferiorly to the heart. Some coils 

 of the intestine are to be seen below the right lobe and leading downwards 

 in the middle line from the median lobe is the epigastric vein. At the 

 lower end of this vein the left division of the bifid allantoid bladder projects 

 sideways : the right division lies behind the vein with a small portion of the 

 rectum visible just behind it. Turning the preparation so as to show the 

 left side, the deeply injected left lung is observed to occupy the space 

 between the left lobe of the liver and the cut edges of the dorsal skin and 

 muscles. The oval yellow testis with black pigment speckled on its surface 

 is placed dorsally, and posteriorly to the lung. At its lower end the vesicula 

 seminalis projects touching the pylorus. The stomach and duodenum lie 

 between the left and median lobes of the liver, and the lung and testis. 

 The stomach tapers to the pylorus. 



A remarkable hermaphrodite Nematode worm, Angiostomum nigrove- 

 nosum ( = Ascaris or Leptodera nigrovenosa), is commonly found in the lungs. 

 Its young pass through the intestine and become sexually mature in earth 

 saturated with decaying animal matter. They are minute compared to 

 their parent and their progeny wander back into the lung. A Trematode 

 parasite, Polystomum integerrimum, with six posterior suckers and two large 

 hooks, frequently occurs in the allantoid bladder : and the multi-nucleate 

 Holotrichous Infusorian, Opalina Ranarum, inhabits the rectum. 



The outer layers of the epidermis are shed periodically a process which does 

 not extend to the cornea of the eye. The skin-glands are of two kinds : (i) 

 mucous glands with clear contents ; (2) glands with granular contents, and prob- 

 ably poisonous like the similar glands in the Toad and Salamander. Chromato- 

 blasts connected to nerves occur in the epidermis, but are most numerous in the 

 outer layers of the corium. The concentration and diffusion of the black pigment 

 within the cells depends on a reflex action of the nervous system set up through the 

 eyes. In this manner the colour of the animal becomes adaptable more or less to 

 its surroundings. The adult frog retains no trace of the organs of the lateral lines 

 present in the tadpole. Besides free nerve-endings, the skin contains special ' touch 

 spots ' flat cells lying near one another, each supplied by a nerve-filament. 



The two olfactory lobes of the brain, generally so-called, are connected an- 

 teriorly by a transverse commissure : each lobe has a second root. They are solid, 

 and it is probable that ' small papillae, situated at their base from which the olfactory 

 nerves spring, and which contain a process of the lateral ventricle, should properly 

 be regarded as the olfactory lobes. These papillae arise prior to the solid anterior 

 prolongations of the hemispheres ' (Balfour). The pineal gland is long and fila- 

 mentous, pierces the cranial roof, and is attached to the skin at the level of a line 

 drawn through the anterior angles of the two eyes. With the exception of the tips 

 of the olfactory lobes, the several parts of the brain contain ventricles. The facial 

 t F 



