COMPAEATIVE ANATOMY. 



211 



killing it in hot water, and then examining it with a Coddington 



Wo procoed to describe the internal organs in detail. Tho 



mouth, sitimtrd I : \M-CH the jaws, loada to a throat which runs a 

 nhort v..i . l.iirkward, thon bonds sharply upward, and then again 



:DIU tho ouUido of thin 



.u-il il.'xiblo horizontal portion, which opens into a globu- 

 lar stoumi-h h.'hind, muscles run to the inner wall of tho shield, 

 uii<l thus provide tho means of sucking up juices. The lower 

 oval and depressed portion of the stomach sends from each of 

 its sides fivo tabes, which bend upwards, and then enter an 

 iiinmlar second stomach, which is situated above tho other. A 

 solid mass lies between tho stomachs, and to this a musclo is 

 attached, which posses through tho central hole of the upper 

 riiitf-liko port ion, to bo attached to the dorsal shield above. 

 This musole not only suspends the stomach, but, by contracting 

 and relaxing, causoa the lower sac to work liko a bellows, and 

 so stirs its contents, driving them through the side tubes into 

 the upper stomach, and thia favours tho functions of digestion 

 and absorption. Almost the whole nutritive process is carried 

 on in the stomach, for only a narrow prolongation of it is carried 

 into the abdomen ; and although this receives ducts from tho 

 liver-fat masses, yet it runs as a short straight narrow tubo to 

 tho vent. Just before the exit, the intestine dilates into a 

 roundish cavity, which is called the cloaca, and receives two 

 ducts, one on each Hide, which arc thought to be excreting 

 organs to remove the azotised products of the breaking down of 

 the tissues of tho body. The ducts, three or four in number on 

 each side, which enter the abdominal portion of the alimentary 

 canal, proceed from the large masses of fatty substance which 

 is collected into a mass of vesicles on either side of tho bag-liko 

 hind segment. Those masses may, therefore, represent the 

 liver, but they no doubt also act as a store of nutriment laid up 

 in tho body. This arrangement would seom to be necessary on 

 account of the precarious nature of the supply of food. These 

 creatures, having to lie in wait for their prey, must bo able to 

 play a waiting game ; and they manifest, by their fierceness 

 when a stray victim falls into their toils, that they appreciate an 

 opportunity which may be long before it is repeated. 



A heart shaped liko that of insects lies immediately under the 

 skin of the back of tho abdomen. It is divided into four chambers, 

 placed in a longitudinal series, and propels the blood forwards. 

 It is contained in a loose membraneous investment, which is 

 called the pericardium. This is a reservoir of the blood received 

 from the system, and it passes from this outer court of tho 

 heart into its four compartments, through small valvular holes, 

 one to each compartment. The large vessel given off in front 

 passes into the cophalo-thorax, and there divides into threo 

 pairs of vessels. The top pair goes to the eyes and mouth 

 organs, the middle pair to tho stomach, and the lowest pair to 

 the legs. The blood from these is collected again, and flows 

 through a long central vessel running along tho floor of the body 

 right to the spinnerets, giving off vessels to the skin viscera, 

 and also sending part of its supply of blood to the pulmonary 

 sacs. After being distributed through the pulmonibranchiae, the 

 blood is collected into a number of vessels which run from 

 these along the sides of tho body, mounting upwards, and dis- 

 charging themselves into the pericardium. The lung-bags con- 

 tain a number of fine leaves which lie close together like tho 

 leaves of a book, and in these the blood is aerated. 



The nervous system in the spider is so concentrated as almost 

 to lose its homogaiujliate character. A small double ganglion 

 rests on the top of the throat, and sends cords to the eyes and 

 jaws. This is connected by two cords, one on each side of tho 

 throat, to a large nervous star-shaped mass, which lies on the 

 floor of the cephalo-thorax, and sends nerves to the legs, while 

 from its hind part a thicker cord passes into the abdomen, and 

 there splits into a number of small nerves which go to all parts of 

 the viscera. Tho great star-shaped mass represents the whole 

 chain of double ganglions, shortened, and compressed into one. 

 It will be seen that this arrangement is very much liko that of 

 the nervous system of the short-tailed Crustacea, or crabs. 



The glands of special secretion are of two kinds. Tho poison- 

 glands lie in the cophalo-thorax, one on each side of tho throat, 

 and in tho upper part of the mandibles. A duct from each 

 gland passes to the point of tho fang, and it conveys a liquid 

 which acts as a rapid poison to insects. 



The silk-secreting organs found in the abdomen consist of a 



great number of tube*, on which rounded clusters of follicles 

 ore found. These hare also dilations in their course. All the 

 tube* end in the spinneret, and the substance they Merete i* 

 Hticky liquid which is squeezed through the open pore* at the 

 ends of tho external organs. It would teem that not only all 

 the threads of each spinneret run together, but that all the 

 strands so formed from all the spinnerets ore united into one 

 cord. As the thread is evolved, the spider usually grasps it 

 with ito two hind feet, which may be either to consolidate it or 

 to draw it out. The Hticky secretion of the web dries on ex- 

 posure to the air. Perhaps, however, in the singular web of the 

 Ciniflo tho separate strands are not blended, for the web has 

 this peculiarity tho lines are composed of one stout thread, 

 encompassed by thinner lax lines, which twist in all directions. 



All the spiders appear to bo silk-spinners, but they do not all 

 apply their arts to the fabrication of snares. Fully one-half of 

 them confine their weaving to the construction of cocoons for 

 their eggs, or for lining or making tubes and tunnels into which 

 they can retreat. Tho little Salticus, which, dressed in a harle- 

 quin suit, courses over the upright walls that the hot summer 

 sun shines upon, springs on its prey, first securing itself by a 

 thread lest it should fall. Those spiders which spin snares, do 

 so according to very different methods. The Agelenidai spin 

 loose, irregular webs over foliage, etc., that communicate with 

 tubes in which the spider lies concealed. The Linyphiidas 

 spin horizontal webs, and stand clinging to their under-sides, 

 back downwards. The chief of all spinners arc the Epeiridce. 

 These spin vertical webs whose lines are all in one plane. The 

 outer framework of these webs is necessarily irregular, because 

 this is determined by the support on which it rests, but all 

 within this is beautifully symmetrical. Lines radiating from a 

 common centre pass to tho cords of the frame, and on these K 

 sustained a close- set spiral line, which runs continuously from 

 centre to circumference, being attached to each radius as it 

 passes them. These lines are very elastic, and will bear a good 

 deal of strain. It is a peculiarity of theee lines that they have 

 on them, at regular intervals, beads of viscid matter, which 

 does not dry in tho air like the silk. A clew runs from the 

 web to tho neighbouring retreat where the spider hides ; and 

 this would seem to answer the double purpose of informing the 

 spider when tho snare is shaken by an entangled insect, and of 

 affording it a way whereby at once to rush upon its victim. 

 When tho insect is powerful tho spider will wait till both ends 

 of tho body are attached to the web, and then, striding over it, 

 it will hold the cords of attachment tense with its wide hind- 

 legs, touch tho insect with its spinnerets, fix a thread to it, and 

 then set it rapidly revolving with its fore-legs, until the insect is 

 completely enswathed in silk, like a mummy. The watch-box 

 of the spider is usually under some leaf, but often it constructs 

 a dome of silk to protect it from tho rain, etc. A most remark- 

 able instance of an animal formed for air-breathing, all of whose 

 relatives live in air, having invaded tho water, is found in the 

 Argyroneta aquatica, which makes a dome under water, and then 

 carries down air, which it places under its diving-bell. 



The scorpion is the type of another group of Arachnida. 

 This creature is much more elongated than the spider, and ita 

 segmentation is very much more distinct, the segmentation of the 

 abdomen furnishing its distinguishing characteristic. The tho- 

 racic shield, which is supposed to represent the dorsal half, is of 

 ciprht rings. About the centre of this are seen two eyes, one on 

 each side, and close to the mid-line, while at the front outer 

 angles groups of simple eyes are found. All the jaws and limbs 

 are supposed to belong to these coalesced segments. Tho next 

 eight segments of the back are as wide as tho cephalo-thorax, 

 while the succeeding six are very narrow, and capable of moving 

 on one another by definite joints in an up-and-down direction. 

 Tho last segment has a hooked spine with its point directed 

 downwards. This is the instrument of attack, and it contains a 

 gland from which poison is ejected in the wound it makes. Thus 

 tho sting, instead of being in the antennal jaw at the head end, 

 ns in tho spider, is placed in the very hind-joint of the scorpion. 

 Tho bases of tho legs almost obliterate the under-segments or 

 sterna of tho fore-part of tho body, but tho succeeding ones are 

 well represented, and through four of them the slits which lead 

 into the eight pulmonibranchiae are cut. Between the black 

 horny back and front pieces of these segments is a white flexible 

 membrane. In tho six tail-pieces the top and bottom ports ore 

 united immovably together. The nature of the limbs is best 



