ECHINODERMS. 133 



nate in csecal vesicles, or pulmonary cells. The right branch is 

 intimately connected with the intestinal veins ; the left branch of 

 the respiratory organ is connected, by means of muscular fibres, 

 with the internal surface of the integument. The form of this 

 respiratory organ agrees with that of Lung, although Holothuria^ 

 breathe water and not air. These parts are very contractile : in 

 a Holothuria that was opened alive they did not cease, as long as 

 life lasted, to force the water in and out by alternate contraction and 

 expansion. But in respiration it is not the contraction of the 

 muscular membrane alone of these branches that acts, but the 

 contractility of the common integument of the body also. This con- 

 tractility of the skin is so great, that occasionally, when the creature 

 is irritated, a portion of the intestines together with the right 

 branch of the respiratory organ is forcibly ejected from the Cloaca. 



In the Sea-urchins VALENTIN considers the ten branched organs 

 surrounding the mouth, first described by TIEDEMANN (and noticed 

 above, vid. p. 132), to be external gills. As internal gills KROHN 1 

 and VALENTIN consider the foliated vesicles, which, in the interior 

 of the shell, are in connexion with the ambulacral tubules : and 

 which have a closely-woven vascular net-work. VALENTIN found, 

 as has been stated, the ambulacral tubules perforated at the extre- 

 mity in Sea-urchins. Through these openings the water penetrates 

 into the vesicles, and the general opinion that the fluid is urged 

 into the tubules from the vesicles and so distends them is not valid, 

 according to VALENTIN, in the case of Sea-urchins 2 . In that of 

 the Star-fishes and Holothurice, where the tubules appear to be im- 

 perforate, it has not been satisfactorily made out to what extent, if 

 at all, the attached vesicles contribute to the respiratory act. 



The organs for propagation are in different families of this class 

 of a different form, but still, as was stated above, have, in the two 

 sexes of the same species, exactly the same form. Hence, it 

 appears that the discovery of the different sexes belongs exclusively 

 to the latest scientific period, since formerly it was believed that all 

 the individuals were of the same sex, either really bisexual or solely 

 female 3 . 



1 MUELLEK'S Archiv. 1841, s. 5, 6. 



2 [This observation of VALENTIN is contradicted by MUELLER, Archiv. 1850, p. 123.] 

 :? WAGNER first discovered the difference of sex in Holothuria tubulosa; then PETERS, 



1840, in Echinus, RATHKE in Ophiura and Sea-stars, &c. 



