Characteristics of Ctenophorae. clvii 



drozoa are ordinarily dioecious. The generative products are always 

 developed between the two layers of the body walls ; and are dis- 

 charg-ed by dehiscence, either into the external medium in which the 

 animal lives^ as in Hydrozoa ; or into the perigastric cavity, to be 

 thence discharged by the digestive tract, as in the two other Classes. 

 Reproduction may take place asexually by gemmation or by fission j 

 and the power of repair and regeneration is very great. The sexual 

 method is very ordinarily accompanied by metamorphosis and meta- 

 genesis ; and the variety of forms found in any one of the compound 

 sj)ecies, is often increased by the specialization of certain zooids to 

 particular functions, so as to be purely agamic and digestive, purely 

 motor as in Siphonophorae, or purely reproductive. 



Class, Ctenophorae. 



Bilateral Coelenterata, ordinarily oval, rarely cestoid in form; 

 the place of the corona of tentacles seen in the two other Classes 

 of Hydrozoa and Anthozoa, is taken ordinarily by a pair of long 

 highly contractile cord-like prehensile organs, the interior of which 

 communicates with that of the system of canals, representing the 

 body cavity, and the exterior of which is armed with thread-cells. 

 Their most distinctive characteristic, and the one whence they take 

 their name, is the possession of four pairs of motor organs, con- 

 sisting of parallel comb-like rows of plates, which work like paddle- 

 wheels in propelling the creatures. They are all marine, and 

 never microscopic in size, nor social, nor indurated by deposit of 

 any kind. They differ from the Anthozoa in having the inter- 

 mesenteric spaces of the body cavity reduced to a system of bilateral 

 canals by the increase of the gelatinous parenchyma ; and from the 

 Hydrozoa, in having the stomach surrounded by, and suspended, 

 though not freely, in a perigastric cavity ; and from both, not only 

 in their well-marked bilateral symmetry, but also in having a com- 

 munication between the external medium and the body cavity, not 

 only through the digestive tract and the mouth, but also by means 

 of a funnel-shaped, and ordinarily bifid canal at the opposite pole of 

 the body. A central nerve-system, consisting of a single or double 

 ganglionic mass, has been supposed to be demonstrable in these 

 creatm'es at the point where the funnel-shaped canal just mentioned 

 comes into communication with the system of perigastric canals, 

 and to send branches in correspondence with the rows of swiumiing 



