208 



ECHINODEEMATA. 



(539.) The generative system of the Holothuria is essentially similar 

 to that found in the Asteridae, consisting of long ovigerous caeca. The 

 germs are secreted in slender ramified tubes (fig. 101, li h) ; these are 

 collected into one great bundle, and open externally by a common canal 

 in the neighbourhood of the mouth not into the oesophagus, as Cuvier 

 supposed, but upon the back of the animal. The generative caeca at cer- 

 tain times of the year become enormously distended, being at least thirty 

 times as large as when not in a gravid state : if examined at this period, 

 they are found to contain a whitish, yellowish, or reddish fluid, in 

 which, in the female, the ova are suspended. In the male a precisely 

 similar structure exists ; but instead of ova, the caeca contain a fluid 

 crowded with spermatozoa during the breeding season. 



(540.) After their escape from the egg, the young Holothuriae have 

 been ascertained to undergo a kind of metamorphosis scarcely less 

 wonderful than that observed in Ophiura and Echinus. In its first or 

 Pluteus condition, the little embryo bears no resemblance whatever to 

 the future animal, but swims vigorously about by the agency of broad 

 membranous-looking expansions that surround the margins of its flat- 

 tened body, wherein the stomach and other viscera are distinguishable 

 (fig. 103, 1, 2). In its second stage of existence it has somewhat the 

 appearance of a polype (fig. 103, 3) ; and this ultimately becomes con- 

 verted into a larva-like being (fig. 103, 4), surrounded with several 

 rows of vibratile cilia, by means of which its progression is accomplished. 

 In the interior of this larva, a set of rudimentary oral tentacula, sur- 

 rounded at their bases by a circle Fig, 104 

 of calcareous spicula, is developed, 

 an alimentary canal makes its ap- 

 pearance, and even the ampullae 

 Polianae are distinctly recognizable, 

 surrounding the position of the 

 future mouth. In its fourth stage 

 of advancement (fig. 104), the Ho- 

 lothurian structure is no longer 

 doubtful, although the apparatus 

 of vibratile cilia still exists upon 

 the exterior of the body. The ali- 

 mentary canal (a) may be seen to 

 terminate in a cloacal chamber, 

 the Polian vesicle (6) is largely 

 increased in size, the calcareous 

 circle (c) around the mouth is much 

 strengthened, the tentacles (d)have 

 assumed larger proportions, and Young Holothuria. 

 even the appearance of the suctorial feet (e) is no longer doubtful ; the 

 longitudinal muscular fasciculi in the integument progressively acquire 



