30 



ECIIINODERMATA. 



filiform, and almost invisible, when empty; but 

 they are susceptible of great dilatation, and 

 are sometimes seen filled with water or dis- 

 tended with smaller animalcules seized as food. 

 Viewed through the microscope these minute 

 animals present very different appearances, ac- 

 cording to the quantity and kind of food con- 

 tained in their crecal appendices, and from this 

 circumstance twelve different species of animal- 

 cules, belonging to six supposed distinct genera, 

 have been formed of the single vorticella con- 

 vallaria. No glandular organs to assist in 

 digestion have been observed in the polygastric 

 animalcules ; and notwithstanding their almost 

 invisible minuteness and the great simplicity 

 of their structure, they appear to be the most 

 numerous, the most active, the most prolific, 

 and the most voracious of all living beings. 

 Very recently, by the aid of an improved mi- 

 croscope made at Berlin, Ehrenberg has been 

 able to detect a dental apparatus in the kolpoda 

 cncullulus of Muller, one of these minute poly- 

 gastric animalcules, which shews a further ana- 

 logy between them and the helminthoid articu- 

 lata. Notwithstanding the number of stomachs 

 in this class of animals, and the infinite variety 

 of prey which commonly surround them, we 

 often observe them devouring animals, which 

 from their magnitude are incapable of being 

 conveyed into these cavities. I have observed 

 a trackelhu, after swallowing several monads 

 which swarmed around it, proceed slowly to 

 swallow down a trichoda, which appeared to 

 be ten times the size of one of its internal sacs. 

 It took about a minute to swallow the trichoda y 

 after having turned it in different directions 

 with its long transparent moveable upper lip. 

 The prey could not be perceived to offer the 

 slightest resistance, while the trachelius, with 

 its upper lip spread over the small anterior end 

 of the trichoda, gradually advanced and ex- 

 panded the short lower lip to embrace it below. 

 The body of the trachelius was much shortened 

 during this prolonged act, being drawn forwards 

 towards the lips, and the animalcule, become 

 slower in its movements, was sensibly distended 

 on one side by this large prey in the intestine ; 

 but in less than half an hour it had recovered 

 its usual lengthened form and gliding move- 

 ments, and was seen to seize again the smaller 

 monads around it. Ehrenberg has figured an 

 enchelys swallowing a loxodes ten times the 

 size of its stomachs even when filled with car- 

 mine, and in the body of the loxodes he has 

 represented nuvicul/z which have been swal- 

 lowed, though several times the size of any of 

 its stomachs distended with sap-green. In the 

 capacious alimentary cavity of the param&cium 

 chrysalis I have found a constant slow revolu- 

 tion of the whole contents, like the cyclosis in 

 the large cells of a chara, and the round sacs 

 appear often to be driven to and fro like loose 

 balls in a sac. Baron Gleichen has figured 

 some of these round sacs of Ehrenberg separate 

 from the animalcules, as a bolus of matter 

 which had escaped per anum. These round 

 transparent bodies are often hurried to one end 

 of the animalcule's body and then to the oppo- 

 site, or spread generally through the cavity, 



and they sometimes join partially in the general 

 internal cyclosis of the abdominal cavity. In 

 many genera of polygastric animalcules a cir- 

 cular proboscis is seen around the mouth, 

 composed of long parallel straight teeth closely 

 applied to each other, which can be extended 

 or retracted, and forms their masticating appa- 

 ratus. 



(For the higher forms of the alimentary canal 

 in all the separate classes of the animal king- 

 dom, see the names of the several classes from 

 the PORIFERA to the MAMMALIA, ANIMAL 

 KINGDOM, and the preceding article DIGES- 

 TION.) 



( R. E. Grant.) 



ECHINODERMATA, (e^w?, echinus 

 IlittMt, corium,} Fr. Echinodermes. A class 

 of invertebrate animals belonging to the di- 

 vision Radiata or the Cycloneurose sub-king- 

 dom. The most familiar examples of them are 

 the common sea-urchin and star-fish. 



In these the skin is covered with prickles, a 

 circumstance from which the class has received 

 its name; but animals of corresponding in- 

 ternal structure, such as the Holothuria, are 

 also comprehended among the Echinodermata, 

 although the skin is destitute of prickles. 

 They are all inhabitants of the sea, examples 

 of them are found in all climates, and the 

 remains of extinct species exist in a fossil state 

 in various mineral strata. 



Naturalists are not agreed as to the limits 

 of this class. Cuvier includes in it two orders 

 of animals; the first provided with tubular 

 retractile organs named feet, the second desti- 

 tute of feet, but allied, he conceives, to the first 

 in other respects. Other zoologists separate this 

 second order of Cuvier from the Echinoder- 

 mata. But in fact these apodous animals, 

 comprehending the genera Molpadia, Minyas, 

 Priapulus, and Sipunculus, are as yet so im- 

 perfectly known, at least as regards their in- 

 ternal structure, that naturalists seem at a loss 

 to discover their appropriate place in the zoo- 

 logical system. In these circumstances we 

 shall confine ourselves to the consideration of 

 the true or pedicellate Echinodermata, of whose 

 systematic arrangement the following is a tabu- 

 lar view. 



Order I. ASTEROIDEA or STELLE- 

 RIDA. 

 Body depressed, divided into rays like a star, 



or at least with prominent angles. Mouth 



inferior, generally no anus. 



a. Holes for the feet disposed in grooves 

 on the inferior surface. 



Genus 1. ASTERIAS, (Jigs. 298 vol. i. 

 7-22.) 



b. No grooves for the feet. 



Genus 2. OPHIURA. Rays simple, elon- 

 gated, cirrhous, with lateral spines. 



Genus 3. EURYALE. Rays long, cir- 

 rhous, divided dichotomously. 



Genus 4. COMATULA. Rays in two 

 sets, dorsal and marginal. The dor- 

 sal rays simple, filiform, cirrhous. 

 The marginal much larger and pin- 

 nated, their inferior pinnules turned 



