433 



L1MULUS. 



LIMULUS. 



451 



very small in the parasitic Bopyrus. In the Cymothoa the seven thoracic 

 and seven abdominal segments are more distinctly characterised." 



With regard to the nervous system and senses Professor Owen, after 

 alluding to Cuvier's description of that part of the organisation in the 

 Common Crab, Mr. Swan's illustrative dissections and beautiful plates 

 of the same, and the able display of the corresponding structure in 

 the Maia by Audouin and Edwards, observes that an analogous con- 

 centration of the nervous system, but with interesting modifications, 

 hag been described by Professor Van der Hoeven in the Limulus, or 

 Kiii2-Crab. the most gigantic form of the Entomostracous tribe, and 

 probably the only existing genus from which we may derive an insight 

 into the organisation of the extinct Trilobitic Crustaceans. Professor 

 Owen was therefore induced to put the well-preserved specimens of 

 Limulta given to the College of Surgeons in London by Mr. Boot, of 

 Boston, United States, into the hands of Mr. Goadby, the anatomical 

 assistant to the college, whose beautiful dissections and preparations 

 will be found in the museum of the College. 



Professor Owen observes, in his Lectures, that the large lateral com- 

 pound eyes of this crustacean are sessile. The cornea, he proceeds 

 to state, is divided into a considerable number of small circular facets, 

 each of which corresponds to an ocellus ; and the optic nerve, after its 

 long course as a simple chord without forming a ganglion, divides 

 near the eye into a pencil of fine filaments, which severally receive the 

 impressions from their respective ocelli, of the aggregate of which the 

 large lateral eye is composed : the two small simple median eyes, 

 which are almost in contact, command the space before the head 

 which is out of th>, range of the large compound eyes. Each simple 

 eye, he further informs us, receives its distinct nerve from the anterior 

 apex of the corresponding cerebral lobe. 



" In the sessile eyes of the Edriophthalma, as, for example, in the 

 Serolit, the inner layer only of the cornea is divided into hexagonal 

 facets, corresponding with the number of the conical crystalline lenses 

 of the compound eye. But in the Trilolitet the cornea presents the 

 same subdivided surface as in the Liiiiulus ; and the position of the 

 two eyes agrees with that of the corresponding compound pair in the 

 large existing Entomostracan. The eyes are more elevated in the 

 Trilobilei. In iheAiaphui caudatui the cornea is divided into at least 

 400 compartments, each supporting a circular prominence; its general 

 form is that of a frustum of a cone incomplete towards the middle 

 line of the head, but commanding so much of the horizon in other 

 directions, that where the distinct vision of one eye ceases that of the 

 other begins. In the Mandibulate Crustaceans, distinguished by 

 having their compound eyes supported on moveable peduncles, the 

 form of the corneal facets varies ; they are square in the river Craw- 

 Fish, hexagonal in the Hermit and Common Crabs. There is a conical 

 crystalline lens behind each facet imbedded in a small vitreous humour, 

 upon which the optic filament expands, and each ocellus is lodged in 

 a pigmental cell, which likewise covers the bulb of the optic nerve ; 

 the cavity containing the compound eye is closed behind by a mem- 

 brane continuous with the inner layer of epiderm, and pierced for the 

 passage of the optic nerve. In the Podophthalmous Criulacea there 

 is generally a spacious furrow or cavity, in which the eye can be 

 lodged and protected, and it i termed the orbit. In one or two 

 species the eye-stalks project beyond the margins of the carapace." 

 (Owen.) 



The same acute observer, speaking of the organs of digestion of 

 these large crustaceans, which form the only genus represented by 

 species which co-existed with Trilobites, remarks that the Limuli differ 

 from all other living Crustacea in their organs of mastication, which 

 are the modified hard joints of the five posterior pairs of legs : the 

 first small pair, serving to bring the food to the mouth, are supported 

 on a rudimental labrum. He refers to the discovery by Mr. Charles 

 Stokes of a distinct subquadrate labrum deeply emarginate anteriorly 

 in Aiapkiu platycephalus [TRILOBITES], and remarks that the nearest 

 approach to this the only known part of the trophi of the Trilobitet 

 seems to be made by the Entomostracous genus Apia, in which 

 however the labrum is truncated. A few of the lowest organised 

 Criutacta, as Caliytu, Nymphon, and Pycnogonon obtain their aliment, 

 he adds, like the Epizoa, by suction. 



After referring to the structure of the month in the Malacostracous 

 Cnutacea, and pointing out that the alimentary canal is most simple 

 in the Suctorial Crustaceans, in which it presents no noticeable differ- 

 ence from that in the Epizoa, the hepatic appendages however being 

 more localised and better developed, he thus describes this part of the 

 organisation in Limulut : 



" In the Limulut the mouth is situated nearly in the centre of the 

 inferior surface of the great cephalo-thoracic segment ; the oesophagus 

 is continued from it in a very unusual course forwards, and expands 

 into a stomach, which is situated at the anterior part of the head. 

 This organ is abruptly bent upon itself upwards and backwards, and 

 if continued by a gradual diminution of diameter, as appears upon 

 an external view, into the intestine, which passes backwards, with a 

 flight vertical bend, to the base of the penultimate abdominal segment. 

 When we examine the interior of the alimentary tract, the distinction 

 between the stomach and intestine is effected, as Van der Hoeven has 

 shown, by a conical valvular pylorus, which projects into the com- 

 mencement of the intestine. The stomach is lined by a vi ry dense 

 and corrugated horny membrane. The hepatic mass which, with 



the generative glands, fills the greater part of the cephalo-thoracic 

 cavity, pours its secretion into the commencement of the intestine by 

 two ducts on each side." (Prep., No. 477, A.) 



In the heart of the Cnutacea, we may trace a gradational series of 

 forms, from the elongated median dorsal vessel, to the short, broad, 

 and compact muscular ventricle in the lobster and the crab ; and in 

 all the Crustacea, as in all the other articulate animals, the heart is 

 situated immediately beneath the skin of the back, above the intes- 

 tinal tube, and is retained in situ by lateral pyramidal muscles. In 

 the lower, elongated, slender, many-jointed species of the Edriophthal- 

 mous Ci*u&tacea the heart presents its vasiform character: it its broadest 

 and most compact in the crab. 



" In this series," says Professor Owen, " we may trace a general 

 correspondence hi the progressive development of the vascular as of 

 the nervous system, concomitant with the concentration of the 

 external segments, and the progressive compactness in the form of the 

 entire body. But there is a remarkable exception to this concomitant 

 progress in the Limulta, indicative, with the general condition of the 

 instruments of locomotion and respiration, of the essentially inferior 

 grade of organisation of that genus, which, as has already been 

 observed, seems to be the last remnant of the once extensive group 

 of Trilobitic Crustacea which swarmed in the seas of the ancient 

 secondary periods of the earth's history. 



" We have seen in the compact and broad existing representative 

 of those extinct gigantic Eutomostracans, that the nervous system 

 exhibits a concentration of its principal central mass around the 

 mouth, analogous to the condition in the common crab, but with a 

 ganglionic double chord continuing from it. The heart however is 

 far from presenting a corresponding degree of concentration : it 

 remains an elongated fusiform tube, extendiug parallel with the 

 intestine from the pylorus to the rectum : it is contained in a peri- 

 cardium with thin membranous walls, formed by the central sinus of 

 the venous system, and it receives the blood from that sinus and 

 from the branchial veins by a series of from seven to ten lateral 

 vertical slits, defended by valves as in the higher Cntstacea. An aortic 

 trunk proceeds from each extremity of this heart. The anterior aorta 

 is the largest, and immediately divides into three branches. The 

 middle and smallest branch passes forwards to the anterior edge of 

 the cephalic shield, following the curve of its middle line, and sup- 

 plying the small median ocelli in its course. The two larger lateral 

 branches form arches, which curve down the side of the stomach and 

 the oesophagus, giving branches to both those parts and to the intes- 

 tine, and becoming intimately united with the neurilemma of the 

 ocsophageal nervous collar. They unite at the posterior part of that 

 collar, and form a single vessel, which accompanies the abdominal 

 nervous gangliouic chord to its posterior bifurcation, where the vessel 

 again divides. Throughout all this course the arterial is so closely 

 connected with the nervous system as to be scarcely separable or 

 distinguishable from it. The branches of the arterial and nervous 

 trunks, which accompany each other, may be defined and studied 

 apart. 



" The posterior aorta is chiefly destined for the supply of the 

 sword-like tail of the Limulus : the first part of its course is wavy, to 

 adapt it to the strong inflections of that appendage. The aerated 

 is mixed with the venous blood in the heart, and is propelled in 

 that mixed condition throughout the body, in the Limulus as in the 

 lobster." 



With regard to the generative apparatus, Professor Owen observes 

 that most of the small JSntomostraca carry the impregnated ova in 

 appended ovisacs, like those of the Lernete. [LERNEAD.fi.] These 

 sacs are not developed, he remarks, in the Limulus, which also differs 

 from the smaller Entomtatraca, inasmuch as the ovarian mass inter- 

 blends its lobes and processes with those of the liver : the oviducts, 

 he adds, form more frequent communications with each other than in 

 the higher Crustacea, but ultimately terminate, like the vasa deferentia, 

 by two distinct but continuous orifices on the back part of the first 

 abdominal lamelliform appendage. 



The Limuli undergo in their youth considerable changes of 

 form. At first there is no sword-like or styliform tail, which in the 

 adult Limulut equals, at least, the rest of the body in length ; their 

 abdominal buckler is rounded posteriorly, and the last pairs of false 

 feet are not developed. M. Milne-Edwards observed this in the 

 embryos on the point of exclusion from the egg, at which period the 

 abdomen supports only three pairs of appendages. 



The species of Limulus inhabit the sea, and sometimes come upon 

 the sandy beaches. They are found in the Indian and Japanese seas, 

 and in the Atlantic, on the coast of North America ; but they do not 

 appear to have a higher range than 44 N. lat, and seem confined 

 to the northern hemisphere. Their food consists of animal substances ; 

 and when stranded, they often bury themselves in the sand as a pro- .. 

 tection against the heat of the sun, which is soon fatal to them. 



M. Milne-Edwards remarks that Leach, it is true, has restricted 

 this generic name of Limulus to those species the whole of whose feet^ n . 

 are cheliform, and has proposed a new genus, under the designation of e( j 

 Tachypleus, for those whose anterior feet are monodactyle ; but he oel ! 

 observes, it is now well known that this last character is only met* ne 

 with in the male of certain Limuli, and does not coincide with othec*.^ 

 peculiarities of structure of any importance, so that it does not appea 



