40 



ECHINODERMATA. 



Tery imple (Jig. 20, e,f, g, A.) 

 At the mouth it is surrounded by 

 the tentacula and calcareous ring 

 already described, it passes back- 

 wards on the right side the whole 

 length of the body (from e to/J) 

 then bending forwards it returns 

 to near the mouth (from /' to g,) 

 and at last runs back again to the 

 posterior extremity (from g to A,) 

 where it terminates in a short and 

 wide cloacal cavity (</), common 

 to it and the respiratory organ, 

 and opening externally at the 

 anus. The intestine is fixed by a 

 mesentery, and the cloaca is con- 

 nected to the parietes of the body 

 by numerous muscular bands de- 

 rived from the transverse muscles. 

 The coats of the canal are thin; 

 Tiedemann enumerates three, an 

 external derived from the perito- 

 neum, a middle which is very vas- 

 cular and contains muscular fibres, 

 and an internal or mucous. In 

 H. tubulosa a small part of the 

 canal near its commencement is 

 wider than the rest, has thicker 

 coats, and is more decidedly mus- 

 cular; Tiedemann regards this 

 part as the stomach. In H. pentactes, the part 

 immediately succeeding the oesophagus and ex- 

 tending nearly to the first flexure, is somewhat 

 cellular and at the same time wider, but thin- 

 ner in its coats than the rest of the canal ; 

 this part is considered to be the stomach by 

 Meckel. 



It is a singular fact, which it appears was 



Fig. 21 



Portion of the shin of Asteriai rubens, seen on the inside and magnified. 

 c f c, peritoneal membrane raised. 



each group the fibrous membrane forming the 

 wall of the body presents on its inside a shal- 

 low pit (Jig. 2 1 , a ; fig. 298, vol. i. e ; Jig. 1 6, s, s,) 

 perforated with holes, through which the tubes 

 communicate with the internal cavity. The 

 tubes are formed externally of the superficial 

 layer of the skin, and are lined in the inside 

 by a prolongation of the peritoneal membrane. 



first noticed by Redi, that several species of This membrane lines the parietes of the body, 



Holothuria, on being taken from the sea and 

 put into a vessel of sea-water, discharge their 

 intestine and part of the respiratory organ 

 through the anus. This operation is effected 

 by repeated contractions of the cutaneous 

 muscles, and some naturalists are disposed to 

 regard it as a voluntary act. 



4. Respiratory organs. The Echinodermata 

 breathe through the medium of sea-water. In 

 the star-fish and urchin the water enters the 

 body, passing into the space in which the 

 viscera are lodged, and this cavity, which, as 

 already stated, is lined by a peritoneal mem- 

 brane and occupies the greater part of the 

 body, is generally regarded as the chief seat of 

 the respiratory process. In the tlolothuria the 

 water is alternately drawn in and expelled from 

 a tubular respiratory organ ramified within the 

 body. 



a. In the star-fish the water is generally be- 

 lieved to enter and issue from the body by 

 numerous small tubes on the surface, which 

 have accordingly been named the respiratory 

 tubes. These are very small, membranous, and 

 in figure somewhat conical (Jig. 298, c, c, 



L615, vol. i.); they communicate at their 

 e with the interior of the body, and are 

 perforated at the summit by an orifice which 

 can be very accurately closed. Most of them 

 are placed in groups or patches, and opposite 



and is reflected over the contained parts; at 

 least it covers the stomach and coeca, and pro- 

 bably also the ovaries and vesicles of the feet; 

 opposite the perforated pits it sends prolonga- 

 tions (6, &,) through the holes into the tubes, 

 as may be easily seen on stripping off a portion 

 of it. 



There can be no doubt that sea-water enters 

 the peritoneal cavity. The animal slowly dis- 

 tends itself with that fluid, and again, but at 

 no stated interval, gives out a portion of it: 

 this is obvious from the fact that the same 

 animal may be seen distended at one time and 

 flaccid at another. Naturalists are generally of 

 opinion that the water enters and issues by the 

 respiratory tubes, and indeed no other orifices 

 have been discovered ; we must, however, 

 freely own that we have never been able 

 actually to observe its passage through these 

 tubes. 



The peritoneal membrane seems to be the 

 principal seat of respiration ; spread over the 

 viscera and the parietes of their containing 

 cavity, and lining the respiratory tubes, it pre- 

 sents a great extent of surface continually in 

 contact with the surrounding medium ; and we 

 have found that a beautiful provision exists for 

 maintaining currents of water along the mem- 

 brane, and thus effecting that constant reno- 

 vation of the fluid in contact with its surface 



