128 LECTURE X. 



length. The integument is also acted upon by transverse fibres 

 which run external to the longitudinal bands ; and such is the irrita- 

 bility of this muscular system, that when the Holothuria is disturbed 

 or captured it will sometimes eject its sand-laden intestine and most 

 of the other viscera by the cldacal aperture, and very effectually unfit 

 itself for anatomical investigations. 



The generative organs constitute, as in other Echinoderms, a very 

 considerable part of the abdominal viscera in the breeding season ; but 

 they present a more complicated form : they consist of a branched sys- 

 tem of long and slender csecal tubes {fig. 67. /"), opening externally by 

 a single common canal, whose orifice is near the mouth. The gene- 

 rative organ of the male Holothuria resembles that of the female in 

 structure ; but the sexes may be readily recognised at the breeding 

 season by the different character of the contents of the tubes, which 

 are white or colourless in the male, whilst the ova present a reddish 

 or yellowish hue. 



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. 



Among the k\v observations which have hitherto been recorded 

 of the development of the Echinoderms, are some which are of great 

 interest. 



According to M. Sars, the star-fish, immediately after exclusion 

 from the egg, presents a depressed, round form, with four short club- 

 shaped appendages at the anterior extremity ; the young animal moves 

 by vibratile cilia with the four arms in advance : at the end of twelve 

 days the five rays begin to grow, and in eight days more the hollow 

 feet appear. The swimming motions have now ceased altogether ; 

 the four original ciliated arms shrink, and in a month they have en- 

 tirely disappeared, and the animal exchanges its binary for the 

 radiated figure. 



The ova of the Comatulse escape from each I'eceptacle, through a 

 round aperture, about the month of July, adhering together in a 

 roundish cluster of about one hundred. About the time of the 

 dispersion of these ova, the minute Pentacrini appear, attached to 

 the stems and branches of corallines, and occasionally to sea-weed. 

 This is attached by a convex calcareous plate, from the centre of 

 which arises the column composed of about twenty-four joints. The 

 capital of the column or body bears five bifurcating arms, which are 

 at first simple, but afterwards acquire the pinnae, and subsequently 

 the dorsal cirri. They further resemble small Comatulae in having 

 a separate mouth and lateral prominent vent. These small Pen- 

 tacrini attain the height of about three-fourths of an inch. 



