ECHINODERMATA. 



41 



which is required in the respiratory process. 

 These currents are produced by means of cilia ; 

 they are more particularly described in the 

 article CILIA, to which we refer the reader. 

 Ciliary currents take place also on the external 

 surface of the body, which probably partakes 

 in the process of respiration ; we have more- 

 over observed them within the tubular feet 

 and on the internal surface of the stomach and 

 cceca ; in this last situation they are probably 

 subservient to digestion, but their use is more 

 fully considered in the article referred to. 



b. The respiratory system of the sea-urchin 

 is very similar. The water enters the body 

 through membranous respiratory tubes, which 

 are collected into ten small bunches (fig. 15, 

 e, e\ situated on the under surface of the 

 animal at the border of the shell, and opening 

 internally by ten perforated pits like those of 

 the Asterias. The fluid being introduced into 

 the peritoneal cavity, is moved along its parietes 

 and over the surface of the alimentary canal, 

 the ovaries and the vascular laminae of the 

 feet, by the action of cilia. Ciliary currents 

 have also been observed on the external surface 

 of the body. 



c. The respiratory organ of Holothuria (fig. 

 34,./, fj h, p. 109, vol. i.) has some resem- 

 blance in form to that of air-breathing animals. 

 It is a very long membranous sac, placed 

 within the body, which opens into the cloaca 

 near the rectum and extends forwards from 

 thence nearly the whole length of the body, 

 either single, or (as in Holothuria tubulosa) 

 divided into two main branches (fig. 20, c, c, 

 cut short, fig. 34, J\f 9 p. 109, vol. i.), which 

 in the vicinitv of the cloaca are joined by a 



short common stem. One of these branches 

 is intimately connected by bloodvessels to the 

 intestine, the other by muscular fasciculi to 

 the parietes of the body. The sac, whether 

 single or bifid, gives off a great many lateral 

 branches, \vhich after successive divisions ter- 

 minate in shut or blind extremities. Both 

 stem and branches contain distinct circular and 

 longitudinal muscular fibres, and contract on 

 being irritated. In the act of respiration sea- 

 water is drawn into and expelled from this 

 organ, and its entrance and exit, which may 

 be readily seen at the cloaca, occur in some 

 species so often as once, twice, or even three 

 times in a minute. The alternate inhalation 

 and expulsion of the fluid are effected partly 

 by the action of the muscular parietes of the 

 body, but principally, it would appear, by the 

 muscular fibres of the organ itself, for Tiede- 

 mann observed the process still to go on, 

 though with diminished activity, when the 

 animal was cut open and the organ exposed. 

 Cuvier states that the sac in some species is 

 without branches. 



5. Vascular system. A system of vessels 

 for the circulation of the blood exists in the 

 animals under consideration. The tenuity of 

 their coats, however, and pale colour of their 

 contents render it extremely difficult to trace 

 completely the distribution of these vessels, 

 and we accordingly find that the descriptions 

 of them given by Tiedemann and Delle Chiaje, 



the principal authorities on the subject, differ 

 materially from each other. According to 

 Tiedemann the proper sanguiferous system is, 

 in its distribution, in a great measure confined 

 to the alimentary organs and ovaries, or to 

 these and the respiratory organ where such is 

 present ; he therefore supposes that the canals 

 which convey the fluid of the feet serve more- 

 over as nutritious vessels to parts of the body 

 also supplied by the sanguiferous system. In 

 short he conceives that there are two systems of 

 nutritious vessels distinct from each other, the 

 sanguiferous system, confined to certain organs 

 already named, and the vessels of the feet, 

 destined to nourish another set of parts ; the 

 vessels of the first system carrying blood, 

 those of the second a nutritious fluid secreted 

 from the blood. Delle Chiaje on the other 

 hand maintains that the two orders of vessels 

 communicate together and form but one sys- 

 tem. From our own observations on the 

 Asterias we are disposed to conclude that the 

 vessels of the feet form a system apart from 

 the bloodvessels, a* is maintained by Tiede- 

 mann ; but there seems considerable reason to 

 doubt whether, as that author supposes, they 

 serve as the nutritious vessels of the parts in 

 which they run ; for even according to his own 

 admirable description it does not appear that 

 they ramify in the tissues, if we except, perhaps, 

 the skin of the Holothuria. Moreover their 

 contained liquid does not present the usual 

 characters of blood or of a fluid adapted to 

 nourish the textures ; it is true there are float- 

 ing particles suspended in it, but the clear 

 fluid when filtered yields no trace of animal 

 matter, but agrees almost entirely in com- 

 position with sea-water ; at least such is the - 

 result of our examination of it in the Aslerias. 

 The vessels of the feet having been already de- 

 scribed, we have here only to give an account of 

 the proper sanguiferous system, following Tiede- 

 mann as our leading authority, but at the same 

 time stating^the more material points in which 

 Delle Chiaje differs from him. 



a. In Asterias- a delicate vessel runs along 

 the upper surface of each of the coeca. There 

 are, of course, ten /such vessels in Asterias' 

 aurantiaca (from which the descrfption is taken) 

 corresponding in number with the creca 

 (fig. 22, v, v). They commence near the 

 extremity of the rays, and, receiving branches 

 from the branches and lobes of the cceca, 

 proceed to the central part of the animal, where 

 they terminate in a circular vessel (i~) which 

 runs round the upper part of the body on the 

 internal surface. The circular vessel also re- 

 ceives ten branches (y, y} from the ovaries, 

 and five from the stomach, which before joining 

 it unite into two (w). The vessels described 

 seem to constitute the venous system, and 

 Tiedemann further supposes that the ccecal and 

 gastric veins convey the chyle or nutritious 

 part of the food from the alimentary organs. 

 The circular vein opens into a vertical canal 

 (h, and^/fg. 12, h\ which descends along the 

 prominent angle between two rays, inclosed 

 in the same membranous sheath with the sand 

 canal already described, and terminates in an 



