212 LECTURE X. 



The generative organs of the Sipunculus are two straight, slender, 

 unbranched, blind tubes, symmetrically disposed, and terminating 

 each by a distinct orifice at the anterior third of the body. 



With respect to the generation of the Echinodermata there seems 

 to be no instances of the mode by gemmation, and very few of 

 that by spontaneous fission. This, however, has been noticed by 

 Montagu in the Holothuria (Si/napta) digitatd, which separates 

 itself without violence into an indefinite number of pieces, apparently 

 by muscular constriction ; and he deems it probable that it thus 

 multiplies itself by natural division. He discovered this Echinoderm 

 on the coast of Devonshire.* In another species of Synapta ( S. Du- 

 verrKBo) M. Quatrefages states, that it, also, is in the habit of casting 

 off segments from the extremity of the body, and that the segments 

 so cast off begin to move, and ultimately acquire the form of the 

 parent animal. \ Professor Miiller has observed that, in Synapta 

 digitata, the head once separated, the fragments of the body, however 

 long they may be, do not break up again, but remain living and 

 moving for a day or more : but the piece with the head redivides as 

 often as it is irritated, and it is only by longitudinally bisecting the 

 head that this process can be finally stopped. | Whoever has ex- 

 amined the Holothuriae in a state of nature must have been struck 

 with their proneness to spontaneously eject the interior organs : Sir J. 

 G. Dalyell, after witnessing this phenomenon, found that they had the 

 power of reproducing the alimentary canal, and he observed another 

 process, in which the parts of the entire body were separated by 

 spontaneous fission. § The brittle-stars {Ophioco7yia) are remarkable 

 for the promptness with which they break themselves up into pieces 

 when laid hold of ; and equally so for the power they possess of re- 

 producing their arms. If a portion of the disc remain attached to a 

 separated arm, the entire individual may be reproduced. But this is 

 an exceptional mode of increase in the Echinoderms. 



The ordinary mode is by the formation of ova and sperm, followed 

 by impregnation. As a general rule, these reproductive elements 

 are respectively developed in distinct individuals. But the genus 

 Synapta offers an exception : here the organs of generation are yellow 

 cords suspended from the pharynx and floating freely in the abdo- 

 minal cavity, where they exhibit vermicular movements. They are 

 covered with a ciliated epithelium : the thick walls of their cavity 

 consist externally of a muscular layer, next of a glandular layer in 

 which the sperm-cells and spermatozoa are developed, and the inner- 

 most layer has a cellulo-glandular structure in which the ova are 



* CLXXX. p. 22. t CLXIX. p. 63. % CLXX. p. 24. § CIX. vol. ii. 



