36 



ECHINODERMATA. 



double rows on the inside of the shell (fig. 10, 

 d.) Monro describes each foot as communi- 

 cating with two of these laminae, and conse- 

 quently every lamina as receiving a branch from 

 two feet; in our own dissections we have al- 

 ways found that both branches of each foot 

 belonged to one lamina. These branches are 

 represented as cut at a in the annexed figure. 



Five longitudinal vessels run down on the 

 inside of the shell, there being one in the 

 middle of each double row of feet (fgs. 10 and 

 14, w); lateral branches go off from these either 

 directly to the feet or to the laminar plexuses 

 when they are present. The five longitudinal 

 vessels descending towards the mouth rise 

 through the dental apparatus named the lan- 

 tern, and open into five sacs or receptacles 

 placed on its upper part, where according to 

 Tiedemann they terminate. Monro on the 

 other hand describes the sacs as communicating 

 together, and states that from them the liquor 

 passes down the sockets of the teeth, and is 

 discharged into the sea. The vessels and la- 

 minae are highly irritable, and by their contrac- 

 tion distend the feet. 



Ten tubular tentacula, similar in structure 

 to the feet, are situated in the vicinity of the 

 mouth (fig. 15, d, d, d.} In Ech. esculentus 

 they are attached to the small calcareous plates 



Fig. 15. 



Part of the inferior surface of tJie Echinus. 

 .a, mouth ; b, b, margin of the inferior open- 

 ing of the shell j e, e, membrane which fills 

 it. 



which are imbedded in the membrane that fills 

 up the aperture of the shell. The plates are 

 each pierced with a hole, through which the 

 tentacula communicate with the canals of the 

 feet. 



In Holothuriae the feet are sometimes scat- 

 tered over the whole surface of the body; 

 in other species (as H.pentactesJ they are 

 placed in five longitudinal and tolerably regular 

 rows ; while in others again they are confined 

 to the ventral surface, as in H. phuntapus, 

 where they form only three rows. The tubular 

 part (fig- 20, 6, &,) is in general very short, 

 and is connected with a simple vesicle inside. 

 The vessels of the feet arise from a circular 

 canal which surrounds the stomach near the 

 fore part of the body. One or sometimes two 

 large pyriform sacs (fig. 34, 6, p. 109, vol. i.) 



open into this canal, and a number of small 

 brown hollow glandular-like bodies are also 

 connected with it. Five vessels issue from it, 

 which run forwards and terminate in a second 

 canal situate immediately within the calcareous 

 ring which surrounds the mouth. This se- 

 cond circular canal is connected with the 

 tentacula, as will be afterwards described, and 

 it gives off five longitudinal vessels which run 

 towards the posterior end of the body, and dis- 

 tribute lateral branches to the vesicles of the 

 feet. Tiedemann regards the fluid contained 

 in this system of vessels as a secretion, and 

 conceives that it nourishes the skin, the mus- 

 cles, and tissue of the feet, besides supplying 

 to the latter the mechanical means of their 

 distension. Further observation would, how- 

 ever, be required in order to determine its true 

 nature, for there is much reason to suspect 

 that the fluid of the feet in other Echinoder- 

 mata consists at least in great part of sea-water, 

 and it is not to be supposed that in the Holo- 

 thuria it should be materially different. 



Under this head we may notice the tentacula 

 of the Holothuria (fig. 34, o, p. 109, vol. i.) re- 

 tracted, as they present a great analogy in struc- 

 ture with the feet. These organs are placed round 

 the mouth and are twenty in number; the ex- 

 tremity of each is formed into a circular sucker 

 surrounded by five or six branched processes. 

 They are hollow, and a great part of them is 

 lodged within the body ; this internal part is 

 long and tapering, and communicates by a 

 small orifice with the anterior circular canal 

 already described, from which the tentacula 

 receive their distending fluid. In the rest of 

 their structure and in their mode of action they 

 resemble the feet. They seem to be very sen- 

 sible, and are probably used as organs of touch 

 as well as prehension. In H. pentactes the 

 tentacula are very large, much larger than in 

 H. tubulosa. 



3. Digestive organs. The digestive appa- 

 ratus is very simple. The sea-urchin and 

 Holothuria have an alimentary canal with a 

 mouth and anus, but in the star-fish there is 

 merely a stomach with ccecal appendages and 

 only one orifice. The cavity in which the 

 alimentary organs and other viscera are lodged 

 is lined with a peritoneal membrane, which 

 being reflected upon them forms their external 

 tunic, and attaches them by a duplicature or 

 mesentery to the inside of the cavity. The 

 Echinodermata are said to live chiefly on tes- 

 taceous mollusca and Crustacea. 



a. In Asterias a short but dilatable gullet 

 leads to the stomach (figs. 16 and 22,^/J, which 

 occupies the central part of the animal, and 

 from the stomach a pair of lobulated coeca (g, 

 g, and i'', g', inflated,) pass into each ray. 

 The stou. ich is connected at various places 

 with the parietes of the body by ligamentous 

 bands; it is thin and membranous, soft and 

 corrugated on the internal surface, receiving 

 externally a covering of peritoneum, and con- 

 taining muscular fibres which are more obvious 

 towards the lower part, when it adjoins the 

 still more muscular oesophagus. Two or more 

 blind sacs (l) t branched in some species, open 



