The birds which have a penis or an erectile and retractile tubercle which serves, are the ostrich, the cassowary, the duck, the swan, the goose, the bustard, the mandou and certain neighbouring species; their hens have a ditoridian organ. The ostrich has a true prong, five or six inches in length, cut by a groove which serves as conduit for the seminal liquor; it is enormous in erection and tongue-shaped. The ostrich hen has a clitoris and coition occurs exactly as among mammals. The swan and duck are also very well provided with an erectile tubercle suited for copulation, and this explains at once the story of Leda, the libidinous reputation of the duck, and his exploits in the barn-yards, veritable abbeys of Thélème.
One can not here describe the copulative organs of arthropodes, comprising insects properly so-called. Enough to note that, however varied their forms, they behave very much as those of superior mammifers and are composed of two essential parts, the penis, sheathed in a penial scabbard, and the vagina, prolonged by the copulative pouch which receives the penis. Fish and birds, lacking external apparatus are reduced to methods which will be later examined. Hermaphrodite mollusks, with a marvellously complicated sexual apparatus, ought also to be studied separately. Finally, the amorous habits of insects form a series of illustrative chapters.
From here, taking count only of exterior male organs or of organs which, internal when at rest, emerge at the moment of coition, one may attempt a vague and new classification of animal series.
1. Presence of penis, or of an erectile copulating tubercle: placentary mammals from man to marsupials exclusively; certain runners and palmipedes; crocodilians, chelonians, certain selacians, arthropodes, the rotifera.
2. Presence of a forked penis: marsupials, saurians, chelonians; scorpionides.
3. Disjunction of the secreting apparatus from the copulating apparatus: spiders, dragon-flies.
4. Absence of penis, copulation by contact: monotremes (omithoryncus), birds, batrachians, crustaceans.
5. No copulation; exterior fecundation of eggs: fish, echinoderms.
6. Indirect transmission of sperm with or without contact (by the spermatophore): cephalopodes, orthoptera.