SEXUAL ORGANS AND TISSUES. 53 



§ 6, Brooding and Young- Feeding Organs. — From very 

 lowly animals onwards, structures are present which are utilised 

 in the protection of the young in their helpless stages. The 

 reproductive buds of some coelenterates become true nurseries ; 

 in one at least of the marine worms [Spirorbis spirillum)^ a 

 tentacle serves as a brood pouch ; various adaptations, such as 

 tents of spines, or cavities in the skin, are utilised in echino- 

 derms. The young shelter under the hard cuticle, or among 

 the appendages of crustaceans, in the gills of bivalves, and a 

 cuttle-fish has been seen with the eggs in its mouth. Among 

 the higher animals, the brood-pouch of Appendicularia (one of 

 the very lowest Chordata), the pockets of not a few fishes, the 

 cavities on the back of the Surinam toad, the pouches of mar- 

 supials, are only a few instances amid a crowd. Sometimes, 

 especially in fishes and amphibians, — e.g.^ the sea-horse, 

 with its breast-pouch, and Rhinoderma darwinii, with its 

 enlarged croaking sacs, — it is the male which undertakes the 

 brooding office. When the young are born alive, the internal 

 female ducts become developed in this connection to form 

 uteri. The ovary appears to serve as a womb in the genus 

 Girardinus among fishes, but it is usually the median portion 

 of the female duct which has this function. In placental 

 mammals, w^here the young are born at an advanced stage, and 

 where the maternal sacrifice is at its maximum, the uterine 

 adaptations become more important and complex. The organs 

 of lactation wall be afterwards discussed. 



