280 



THE POPULAE EDUCATOR. 



an aberrant species of the class than its proper type. Its 

 residence in moist earth has imposed upon it a different means 

 of locomotion, and also of respiration, to the more typical 

 examples of the annelids. The proper typo of the class is 

 rather found among those sea-worms which fishermen find in the 

 sand at low-tide and use as baits for fish, and to which the 

 names of lob-worm and lug-worm are applied somewhat indis- 

 criminately. One of these is the Nereis, represented in the 

 engraving. This worm consists of a great many segments; unlike 

 the earth-worm the head is very distinct, and is furnished both 

 with antennas or feelers, and eyes, though the latter are of very 

 simple structure. The orifice of the mouth opens on the under 

 side of the body. The worm appears in its ordinary condition to 

 be a very innocent, inoffensive, and defenceless animal, like the 

 ordinary earth-worm ; but this appearance is deceptive, and 

 only arises from the fact that the powerful jaws are concealed. 

 If the animal be handled while alive, it will suddenly evert and 

 protrude a formidable proboscis armed with a large pair of 

 horny jaws, and with these it will attack the fingers of its 

 captor. The proboscis while at rest is retracted in the same 

 manner as the finger of a glove might be withdrawn by 

 pulling the end in from the inside. These retractible trunks 

 are very general throughout the families of the sea-worms, and 

 though in the Nereis the proboscis is only armed with one pair 

 of jaws, in some it has seven, eight, or nine jaws. Thus, 

 singular as it may appear, we find that the bilateral symmetry, 

 which is so strict in other parts of the body, is here interfered 

 with, for in some species three jaws are found on one side, and 

 four on the other ; and in other species there are four jaws on 

 the right-hand side and five jaws on the left. Besides these 

 jaws which play laterally, other instruments are sometimes 

 attached to the trunk. One species has a circlet of cartila- 

 ginous beads round its proboscis, and another has a number of 

 horny plates so arranged as to form a file. In others the 

 trunk is quite unarmed, and must act as a flexible lip to suck in 

 substances. 



With regard to the remainder of the segments behind the 

 head and proboscis, they are almost exactly alike, the external 

 appendages being locomotive organs, and more or less developed 

 gills. 



In the case of the Nereis, the gills are not well developed, 

 consisting of slightly protruded thin membranes where the 

 breathing function is localised ; but in nearly allied forms, these 

 gills are developed into branched bundles of vessels, or into 

 plates or leaves, in the substance of which the blood enters, 

 and becomes exposed to the influence of the surrounding water 

 through their very thin walls. There is thus considerable 

 difference in the nature of their organs, which will be best 

 understood after we have described the general circulation in 

 these animals. It is sufficient here to say that the gills or 

 tufts of respiratory vessels are, when present, always protruded 

 from the back or upper side of the animal, and are sometimes 

 not developed in every segment, but confined to certain regions 

 of the elongated body. So in the common lob-worm (Arenicola) 

 they are limited to the sixteen middle segments of the body. 

 The locomotive organs which are most effective are bundles of 

 hairs or bristles which stand at the ends of protrusions from 

 the body wall, and which can be more or less retracted into the 

 flexible papillae which bear them. These bundles of bristles 

 are always paired, and sometimes there are two pairs on each 

 annulus. They are brush-like oars, which the animal plies with 

 such regularity one after the other in succession, that the 

 general effect produced to the eye is as of a series of waves 

 following one another from head to tail. Hence the simile 

 often applied to these animals of a galley with its bank of 

 oars, or of an elongated cutter with a multitude instead of 

 eight oars, is hardly a good one, since the oars of these all 

 play in unison, while the brushes of the sea-worm play suc- 

 cessively. Nevertheless, the remarkable order which the simile 

 suggests is well maintained. These organs are, of course, very 

 different from the jointed limbs of the arthropoda, and cannot 

 be applied to points of resistance with the same definiteness 

 and accuracy. Yet they are not ill suited to the wants of the 

 animals, for these are always found among the sand and mud of 

 the bottom of the sea, and their false limbs are equally effective 

 against water, mud, and sand. Besides the bunches of bristles, 

 there are fleshy appendages called cirri, which seem to have 

 also a locomotive function. The relation of these to one another 



is well seen in the ventral feelers shown in the section of a 

 segment of the body of the Eunice. 



The food canal of these animals runs in a straight or slightly 

 flexuous course from head to tail. There is sometimes a mus- 

 cular gizzard, and generally the tube is more or less sacculated 

 that is, it consists of alternate enlargements and constric- 

 tions, the enlargements usually corresponding to one or more 

 of the outer rings of the body. From the outer wall to the 

 constrictions run partitions which divide the body cavity into a 

 number of chambers. These partitions are not complete, but 

 are perforated so as to allow of the passage of the fluid of the 

 cavity (called the chylaqueous fluid), which flows rapidly from 

 one chamber into another with the movements of the creature. 

 In some the body cavity, or space between the food canal and 

 outer skin, is large ; but in some it is but small, and in earth- 

 worms it is almost obliterated. In the leech it is absolutely 

 lost, so that the same network of vessels which runs round the 

 stomach, and sucks and absorbs the alimentary liquids into 

 the blood, also supplies the integument with blood, and there 

 exposes it to the influences of the air. 



From the foregoing sentences the intelligent reader will 

 have gathered that in the annelids there are two distinct vital 

 fluids which are shut off from one another : 



1. The fluid which occupies all the space between the food- 

 tube and body-wall, which is of watery consistence and pale 

 colour, though containing albumen and little roundish bags 

 called corpuscles. 



2. The fluid contained in the blood-vessels, which has usually 

 no corpuscles, and is of a dark red or green colour. 



Though at first one might have supposed the last-named 

 was the blood proper corresponding to the blood of the higher 

 animals, on account of its being contained in definite veins 

 and arteries, yet the real representative of the blood is the 

 first-named fluid, and the vascular system corresponds to the 

 ambulacral, or water-vascular system, which we have described 

 as found in the annuloida. This system is very probably repre- 

 sented in insects by the tracheal system ; a system which we 

 must describe hereafter, and which is applied to an utterly 

 different purpose. Whatever be the homology, or structural 

 affinity, of this vascular system in worms, it attains in them 

 a high degree of perfection and complication. 



In the Eunice, which may be taken as a type of the circula- 

 tion in those worms whose integument is distinct or not 

 adherent to the food canal, the arrangement of the vascular 

 system and the course of its contents are as follow : Two main 

 vessels run along the upper side of the intestine, and receive 

 the blood and the fluids added from the aliment from the net- 

 work of vessels which invests that tube. At the point where 

 the dilated throat joins the intestinal tube a large vessel runs 

 round the alimentary tube, while the two vessels before named 

 are united into one large contractile vessel, and thus continued 

 forward towards the head. The large single vessel does not 

 adhere to the throat or pharynx, but acts as a heart to propel 

 the blood received, not only from the intestine, but also from a 

 vessel which runs along the integument of the back. The blood 

 thus derived from the system, both alimentary and intestinal, 

 is forced by branches to the head and also round into a vessel 

 which runs along the floor of the body. This ventral vessel, as 

 it is called, gives off at each segment a lateral branch on each 

 side, which is bent into a muscular loop, which acts as a special 

 heart to drive the blood to the network of vessels lying undei 

 each tuft-like gill. After being aerated in the gills the blood is 

 returned to the main dorsal vessel by ducts, which are sustained 

 by the partitions which join the body wall to the intestinal 

 wall. In the case of the lob-worm (Arenicola) the gills are. 

 supplied from vessels which branch off from the main trunk 

 running along the top of the intestine, and they return the 

 blood to the great dorsal vessel, which is situated in the mid- 

 line of the integument. The gills of this creature are beautiful 

 objects under the microscope, although they appear to be but 

 confused tufts of vessels to the naked eye. Although these 

 tufts of vessels are so very delicate that the blood shines 

 through them, and as indeed it is necessary they should be for 

 the function of respiration to be accomplished, yet they have 

 voluntary muscular fibres running round them. This is mani- 

 fest, not only from the revelation of the microscope ; but also 

 from the fact that the animal can empty and retract any or all 

 of its gills at pleasure. In this case the gills are little else than 



