372 ZOOLOGY SECT. 



-haemoglobin almost identical with that which gives the red 

 colour to the blood of the higher animals. 



The enteric canal is. as already mentioned, surrounded at 



*/ ^ ' 



its oral extremity by the circlet of tentacles, and within these, when 

 they are fully exserted, is a narrow peristome with the mouth in 

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

 comes inverted, so that peristome and tentacles become enclosed 

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

 Surrounding the (esophagus, which lies immediately behind the 

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

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

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

 ambulacral vessel, blood-vessel, 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 coelome, 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 (rcsp.). 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 ciliated funnel opening into the ccelome. Besides having 

 to do, most probably, with the respiration of the ccelomic fluid 

 and with the excretion of waste-matters, these organs also have a 

 hydrostatic function ; it is through them that, when the tentacles 

 are withdrawn, the overplus of fluid which would impede the 

 process is got rid of, and through them, in like manner, that the 

 quantity is again increased when the tentacles are protruded 

 again. 



Reproductive Organs. The Sea-cucumber, like the Starfish 

 and Sea-urchin, has the sexes separate. Ovaries and testcs (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. 



The early stages of development are very similar to those of the 

 Starfish (p. 358). The bilateral larva, however, assumes a shape 

 somewhat different from the Bipinnaria of the Starfish, and is 

 termed the Auricula rift (Fig. 315) : 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 

 Bipinnaria, persist to the adult condition. 



