532 GENERATION. [CHAP. XXXVI. 



[Reproduction in the class Acalephce takes place almost entirely by the for- 

 mation of ova. It has, however, been shown by Professor Huxley, that some 

 multiply by gemmation as well as by the production of ova (Diphyidae). 

 Some of the species are unisexual, and others bisexual. The genital organs 

 are only developed at certain periods, and the male and female elements are 

 brought into contact through the influence of the water in which they swim. 



In the Echinodermata, fission has only been observed to occur in one class 

 (Holothuria) ; and the generative function, which is developed in this class 

 to a great extent, is carried on almost exclusively by the production of ova. 

 The sexes are distinct, but the ova are impregnated without sexual inter- 

 course. In some there is a proper efferent duct ; but in others the elements 

 pass into the respiratory cavity, and thus escape from the body. 



Among the Entozoa great variety is met with in the arrangement and 

 character of the generative organs. Almost all the animals of this class 

 possess true generative organs, and multiply by means of ova, but in many 

 of them fission occurs ; as, for instance, in the tape-worm ; but it is 

 worthy of remark, that the entire animal is not produced in this process. 

 The segments, however, which have been separated continue to live. As 

 already mentioned, the Echinococcus multiplies by the formation of buds. 



Some of the Entozoa are unisexual, and have the power of self-impregna- 

 tion, and some are bisexual. 



The Annelida reproduce by sexual apparatus, and in some instances, as 

 already referred to, by transverse fission. In the latter case, the different 

 organs, including the tentacles and eyes, are developed before the new animal 

 is separated from the old one. This mode of multiplication, however, only 

 continues for a certain time ; at length it ceases ; genital organs, which 

 before could not be distinguished, are developed, and ova are formed. The 

 Hirudines and Lumbrici are hermaphrodite, but copulation is necessary for 

 impregnation to take place. 



Amongst the lower Mollusca, the sexes are sometimes united in one indivi- 

 dual, and sometimes distinct. There are no copulatory organs, so that the 

 water forms the medium by which the spermatic particles are conveyed to 

 the ova. Amongst the Tunicata, multiplication also takes place by gem- 

 mation. 



Of the higher Mollusea, some are hermaphrodite, and in others the sexes 

 are distinct. Many families are characterised by the possession of what has 

 been termed an hermaphrodite gland, which is almost always imbedded in 

 the substance of the liver. This gland consists of numerous radiating and 

 branched cceca. Each coacum consists of an external and internal sac folded 

 within the first. Ova are produced by the external sac, and spermatic par- 

 ticles by the internal one. Excretory ducts pass off from these organs, and 

 terminate in two tubes ; the one corresponding to the Fallopian tube, the 

 other to the vas deferens. Besides this apparatus, there is also another organ 

 connected with the excretory duct ; this is the albumen gland, which fur- 

 nishes a secretion in which the ova become imbedded as they pass towards 

 the external orifice. This curious arrangement may be well seen in the 

 common snail. 



Into the same cavity or cloaca in which the genital ducts terminate, is 

 found the opening of another very remarkable organ the dart sac in which 

 a hard and excessively sharp-pointed, and sometimes toothed, calcareous body 



