DIOUTK. 



ACAl 



M 



M'K'UTK, one of a group of miner*!* of doubtful identity, 

 oompoMd of newly SO per cent, of (ilia*, with alumina, lime, coda, 

 putuh, and water. 



from iuaA^n, a n. :/. a cliws of 



marine invertebrate p*wi r belonging to the sub-kingdom Katliaia. 

 It u now made to include a large number of animal*, of which the 

 (no* ifnl<ua of Linnicu* may 1 taken a* a type. 



..com Htd**t was placed by Linwviu in the woond section of 

 hi* Vtrmu*. viz. Molltuca. The Mollutea were divided into six 

 Motion* in the 'Systems Nature; ;' and in the but of these, c.. 

 of thoee molluscous forms which had a central mouth below, Mnliwa 

 stood a* the first genus, followed by Ailrrini and Erhinui. Th. third 

 section of Verwu* (Tatatra), with Chiton at ita head, immediately 

 followed. In this arrangement Mliua came between Xertit and 

 Atteriat ; but in the body of the work it stands between Sepia and 

 Attrria*. 



The following is the I.inmran definition of the genus Medina : 

 B"dy gelatinous, orbiculate, depressed. Mouth beneath, central. 



The genus contained 12 species, and these consisted not only of 

 true Hedmtr, but of such genera as Porpita and Vililla. 



The AealffAtr of Cuvier (his third cluss of Zoophyte*) comprehend, 

 to use his own terms. Zoophyte* which swim in the sea, and in who.-.- 

 organization may be perceived vessels, which in truth ore most 

 frequently nothing but productions of the intestines, hollowed in the 

 parenchyma of the body. 



er's first order of Acaleplur, or Sta-XeHla, consist* of the 

 Simple Acaltpkte, which he characterises as floating and swimming in 

 the sea by means of the contractions and dilatations of their body, 

 their substance being gelatinous, without apparent fibres. The sort 

 of vessels which are seen in some are hollowed in the gelatinous sub- 

 stance ; they often visibly come from the stomach, and do not give 

 place to a true circulation. 



The genera contained in this order are the great genus Medina, 

 I. inn., with it* subgenera Porpita and \'cl<tla. 



..rest genus Median is characterised as having a disk more or 

 less convex above, similar to that of a mushroom, and culled the 

 umbrella. Its contractions and dilatations concur to the motivity of 

 the animal. The edges of this umbrella, as: well as the mouth, or the 

 suckers, more or less prolonged into pedicles, which take it* place, in 

 the middle of the lower surface, are furnished with tentacles of very 

 different form and size. These different degrees of complication have 

 given rise to very numerous divisions. 



The Aracknodermala form the second class of M. de Blaiuville's 

 -laria. He observes that this class corresponds exactly to the 

 genus Medina of Liuiueus. 



The following is M. de Blainville's definition of this genus : 



Body free, regularly oval or circular, subgelatinous, covered with 

 on extremely fine skin, which is but little or not at all distinct, 

 sustained or not by a solid subcartilaginous part, and provided with 

 very diversiform radiated appendages. 



Intestinal canal limited to the stomach, and provided with a single 

 oiUae 



Ovaries multiplied, radiated, and opening in the interior of the 

 stomach. 



M. de Blainville goes on to state that their form, which is regular, 

 is nearly always circular (the VeUIltr alone being oval), sometimes 

 discoid or spheroidal, but most frequently hemispherical, which 

 cause* them to resemble our umbrellas, and has given rise to the 

 distinction of their body by that name. This body is son 



; d in .iddition, in its circumference, with more or less long 

 cirrhi, to which the name of tentacles, or better, of teutaculiform 

 cirrhi, has liven given. 



Tin- lower rarnoe of the umbrella, he observes, is sometimes entirely 



naked, but in other coses is provided with numerous and dispersed 



tentoculiform suckers, as in tin- /'..//, ir.r and I'drttir, or else with \.T\ 



form appendages, capillaceous at least at mity. which 



zoologist* have termed arms, win-* ..-uinati I /V.i.-Aicfeotti 



which they have given to some specie*. These appendages or arms 

 ore sometime* free from their base, but in other cose* ore united, 

 which unity produces a sort of peduncle, which has originated 

 the designation of Ped*nc*lnt ,1 for those species that are so pro- 

 vided. In the mi. Idle of the lower surface of tl,, umbrella 0) 

 Mfdiutr U sometimes a species of peduncle formed by a proboscidi- 

 form prolongation of the Iniccal orifice, and they are then called 

 Probotcideaiu ; but in the greater number of coses, the middle of 

 th.- lower part of the umbrella is occupied by a more or less con- 

 siderable moss, attaching itself to the body by four root*, in the 

 f .1111 of a cross, so ax to divide the buccal orifice into four semi 

 lunar parts. Thin ]ieduucle, terminated by more or less nunu-rous 

 r.ipi ll.t> vim- has caused the name of PedtmaUu' 



Polyiti.matitia, to be applied to those Medina which are provided 

 with it 



<.rst siuVlivmoiiK of the Ararlinodrrmata established by M. do 

 Blainville depends on the existence or absence of a solid piece fur the 

 siip|irt of tin- tiiuliiv ||:i ,,r |N,,|Y .if tin- animal, and consists of thu 

 which are provided with that support, and ..f the 

 Putmoyratla, whi-h are without any mieh support. 'I 

 observes M. de Ulainvilln, are further distinguished by the very 



different nature of the appendage* with which the umbrella is 

 furnished on the buccal surface. 



The difficulty of examining from the very nature 



of the:: in iii spirit 



great It is ; their 



organization remained for a lung time in ..i - much 



. t<i it still remains to he cleared up. To observe them with 

 anything like a satisfactory result, they must be MI. died on the spot, 

 and while they are olive ; and thus it ;- that several 

 their oigaiii/.ation and habit ..hove 



only lately been cleared uji. The possibility of fairly prescrv ing them 

 in spirit is shown in the museum of the lioiul College of S 

 London, where, in the depa.tmcir the first il. 



of the Preparations ,,i' N:,t. Hist., in spirit, sevend of the Aeidephans, 

 (No. 64 to 73 A, both im-lusivet are to Iw seen so preserved. In the 

 following remarks the stnicture of the typical Medtuae will be more 

 especially referred to. 



The disk presents a uniform cellular appearance internally, and the 

 cellular substance is very soft. In its mass no til .re has hitherto, we 

 believe, been traced, and indeed the quantity nf solid mutter in the 

 whole animal must be \ cry small. Those who have taken Mediate 

 out of the sea, and laid them upon a dry board or dry stone, must 

 have observed how soon they sink into a sort of dclicpiescence, 

 Spallanzani came to the conclusion that the sea-water p. i 

 organic texture constitutes the greater part of tin- volume in tin' 

 MttluMr, some of which when newly taken out of the sea weigl 

 ounces, though their dri. d remains gave a weight of little more than 

 6 or 6 grains. A fine muscular membrane appearing, when examined 

 with a magnifying glass, to be composed of numerous fleshy fibres 

 disposed in small bundles, radiating as regards the axis of the Mrdtua, 

 and adhering closely to the gelatinous substance of the disk, may be 

 seen in some species stretching over a given extent of tin- i 

 of the umbrella, a little within its outer margin. Portions of the disk, 

 or umbrella, have been cut from these animals u 1. 1 -: ' h< 

 those portions which hail no part of this muscular membrane av 

 to them exhibited no signs of motion; in those, on the other hand. 

 whose connection with the muscular membrane was preserved, the 



cal contraction and dilatation were continued for 

 Those PulmiHji-iiilii which have cilia around their margins hai< 

 circular vessels running along their bases, and m-t of the pi 

 and extensile tentacles and fil i canals with con- 



tained fluids at their roots. If these cilia may be regarded, an- 1 

 doubtless may be, as one of the causes, and a principal one. of loco- 

 motion, the pensile tentacles of the Medutrm\ be viewed as ai,> 

 t least to that faculty, though they proliaMy are principally employed 

 as nutrient organs. They are hollow and simple, and app 

 Increase in their extensibility in proportion to their connexion with 

 the appendages of the digestive mvitics, or when furnished with a 

 at their base. Suckers are found nt the. extremities and along 

 the sides of these tentacles in several of the genera, so as to . 

 them more securely to catch the floating destined prey, or t-i a- 

 anchoring the .)/../, -./ when it would rest, as we have reason to 

 believe it occasionally does. 



fferrout .*'"" ,m,l .vntet. We are not aware of any .|iiit- 

 factory demonstration of a nervous system in the Acalephans. l>r. 

 Grant indeed (' Zool. Trans.,' vol. i.) notices a structure in I 'in/i'j,/,- 

 which in his opiirion can only belong to that system ; but Kschseholt/., 

 whose labours in investigating the organ his class w. . 



small, failed to discover nerve- in the largest which he exio 

 That 1 1, - .1 question. Klirunberghascndcat 



to show that Mnliua a -wes eyes in the form ot 'small red 



i isible on the surface of the eight brown masses win 

 round the circumference of the umbrella ; and he has computvd the-e 

 Ho-eull. huse of certain Huliftrn om\ /;';i/.,i././vr.-,i. He con 



siders the glandular body at the base of the pc<i > optic 



ganglion, and notices its connection with two filaments that dec 



the middle of their course ; and he views these as , 

 part of a in . nited, for the greater part ol 



directly along the hoses of th. row of tentacles surrounding the 

 umbrella, anil no forming a sort of outer wall ot the circular vessel or 

 appendage of the intestinal cavity which run- round the margin of 

 the umbrella. He also describes another < 



ot four ganglion-like masses. These he states .-. d round 



the mouth, ami to be each connected with a corresponding group of 

 tentacles. 



But the general opinion seems to be that touch is the only sense 

 possessed by the Acalephans. as far as proof has hitherto gone. That 

 they ore sen ht, though the evid. - nr of their 



possessing si L . K> called nun not lie deemed conclusive, will 



Iw generally admitted. It is said that some ,.!' th. -smaller ti il .. 

 been known to shun a bright light, and to sink into deep w 

 ovoid it 



The chief seat of the touch appears to be in the tcntarnla and 

 with which the maj : ::, ..i V c furnished. Mam . -t 



a-, we have omveh i make no sign when wounded 



in the umbrella or disk. 



-/.' in. In most of the s| ie- of f'nlii""/:'"il' ''' 



distinct inuseului ; . whi.-li jn-uKcut Uie j euliai markings 



