110 COMPARATIVE ANATOMY. 



within every hermaphroditic animal two essential elements 

 of generation are contained, few animals possess the power 

 of self-impregnation. Such are Hydra, Holothuria, solitary 

 Ascidians, and, probably animals developing by encapsula- 

 tion (q. v.). 



The union of cell-products results in impregnation of ovum. 

 This may occur outside the body, as in certain annelides, 

 molluscs, and fishes; or just within the mouth of the ovi- 

 duct by adaptation of parts (coitus), as in the majority of 

 animals. Coitus may be simply the contact of the mouths 

 of the seminal and oviducts, or penetration of the latter by 

 means of an intromittent organ (penis). 



PROTOZOA. Animals, so far as known, hermaphroditic. 

 In Tethya spermatic filaments and ova lie side by side,; no 

 ducts. In Paramecium the ' nucleus' represents the ovary, the 

 'nucleolus' the testicle; no ducts. Reciprocal transmission 

 of spermatic filaments takes place through the oral openings. 



RADIATA. The sexual organs resemble one another so 

 closely that, excepting at times of procreation, they cannot 

 be distinguished, and even then only by their minute struc- 

 ture. They may be developed in the indifferent tissue of 

 body, as in Hydra, or appended to the inner edge of the body, 

 as in Actinia. Their contents, as a rule, escape by the diges- 

 tive cavity. Polypi may be hermaphroditic, as in Hydra, or 

 unisexual, as in Actinia. In Acalephse the genital organs are 

 not developed, except at the time of procreation, and this 

 period is very brief. The organs consist either of elongated 

 pouches or ribbon-like bands, which are situated in different 

 parts of the body. In the first, sperm and eggs escape 

 through particular excretory canals ; in the second, they es- 

 cape directly outward from the ovaries or pass through large 

 cavities which communicate. In Hydroida the organs are 

 rarely found upon hydroid stock, as in Rhizogeton ; more often 

 they are arranged in the form of brilliantly-colored rays be- 

 neath umbrella-like expansion of medusa bud. 



Echinodermata. Generally unisexual. Holothuria and 

 Synapta exceptions. In Toxopneustes (sea-urchin) the posi- 



