234 THE EVOLUTION OF SEX. 



on the female's back, a sort of erysipelas sets in, and each ovum 

 becomes surrounded by a skin-cavity in which the tadpole develops. 

 After the process is over, the skin of the back is renewed. In other 

 cases this mode of carrying the ova becomes somewhat more definite; 

 thus in Notodehphys and Nototrema the eggs are stored in dorsal 

 pouches. Nor are the males without their share in the task of parent- 

 age. In the obstetric frog {Alytes obsietricans), the male helps to 



FlG. 90. — The female Nototrema marsitpiatum, —an .miphibian, 

 with eggs in a dorsal sac, which is shown partly un- 

 covered. — From Carus Sterne, after (lunther. 



remove the eggs from the female, twists them in strings round his 

 hind legs, and buries himself in the water till the tadpoles escape and 

 relieve him of his burden. In Rhinoderma darwinii, the croaking sacs, 

 which were previously used for amatory calling, become enlarged as 

 cradles for the young. 



Among fishes, parental care is largely in abeyance, and there are 

 only slight hints of anything in the way of incubation. In a siluroid 

 fish (Aspredd), the female deposits her ova and lies upon them till they 

 become attached to the spongy skin of the belly, very much as hap- 

 pens in the dorsal attachment of the Surinam toad. After hatching, 

 the skin excrescence is smoothed away. In Solenostoma (allied to 

 pipe-fish) the ventral fins unite with the skin to form a pouch in which 

 the eggs are retained. In other cases, it is the male which incubates 

 or cares for the ova. Not a few form nests, as in the stickleback, over 

 which they keep a jealous guard. In some species of Arius the eggs 

 are carried about in the pharynx; while in the sea-horses a pouch is 

 developed on the posterior abdomen. 



Among invertebrates, brood-chambers or cradles for the young 

 are not uncommon. The capsules of hydroids, the tent of spines on a 



