396 ZOOLOGY SECT. 



the centre. When the tentacles are retracted the peristome be- 

 comes inverted, so that peristome and tentacles are enclosed 

 within a chamber, the buccal chamber, into which the mouth leads. 

 Surrounding the oesophagus, which lies immediately behind the 

 buccal chamber, is a circlet of ten circum-cesophageal ossicles, five 

 ambulacral (rad. oss) in position, and five inter-ambulacral (inter, 

 oss). Through each of the former pass the corresponding radial 

 ambulacral vessel, haemal strand, and nerve. The alimentary canal 

 itself is a simple cylindrical tube, only indistinctly marked out 

 into oesophagus, stomach (stom), and intestine. It forms several 

 coils within the ccelome, to the wall of which it is attached by a 

 thin membranous dorsal mesentery, and terminates behind in a 

 comparatively wide chamber, the cloaca (cl). 



Opening into the cloaca is a pair of remarkable organs of 

 doubtful function, the so-called respiratory trees (resp). Each of 

 these, beginning behind in a single tubular stem, becomes elabo- 

 rately branched in front, some of the branches reaching nearly to 

 the anterior end of the body-cavity. Each of the terminal branches 

 ends in a small enlargement or ampulla. Besides having to do, 

 most probably, with the respiration of the ccelomic fluid and with 

 the excretion of waste-matters, these organs have a hydrostatic 

 function ; it is through them also that, when the tentacles are with- 

 drawn, the overplus of fluid which would impede their retraction is 

 got rid of, and by their means, in like manner, that the quantity is 

 again increased when the tentacles are protruded again. In all 

 probability it is through the permeable walls of these organs that 

 additional supplies of sea-water are received into the ccelome, and 

 thus reach the ambulacral system through the perforated end of 

 the madreporic canal. 



Reproductive Organs. The Sea-cucumber, like the Starfish 

 and Sea-urchin, has the sexes separate. Ovaries and testes (gen. gl) 

 are very like one another, and consist of bunches of tubular follicles, 

 which communicate with the exterior by means of a duct opening on 

 the dorsal surface some little distance behind the oral end (gen. ap.). 



The early stages of development are very similar to those of 

 the Starfish (p. 381). The bilateral larva, however, assumes a shape 

 somewhat different from that of the Asteroidea, and is termed 

 the auricularia (Fig. 348) : it has a number of short processes 

 developed in the course of the ciliated bands. The larval mouth 

 and oesophagus, instead of being abolished as in the case of the 

 Starfish, persist to the adult condition. 



4. THE CRINOIDEA. 

 A Feather-star. Antedon rosacea. 



General External Features. In the Feather-star (Fig. 

 335), as in the Starfish, there are to be recognised a central disc and a 



