ECHINODERMS. 133 
nate in cxeal 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 Holothurie 
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. 
Tn the Sea-urchins VALENTIN considers the ten branched organs 
surrounding the mouth, first described by TreEDEMANN (and noticed 
above, vid. p. 132), to be external gills. As internal gills Kronn} 
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?. In that of 
the Star-fishes and Holothurie, 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 *. 
1 MUELLER’S Archiv. 1841, 8. 5, 6. 
® [This observation of VALENTIN is contradicted by MUELLER, Archiv. 1850, p. 123.] 
3 WAGNER first discovered the difference of sex in Holothwria tubulosa ; then PETERS, 
1840, in Hehinus, RaTHKE in Ophiwra and Sea-stars, &e. 
