Annelid

“ And if he pass not near enough for that deadly floral embrace, those pretty crimson tubercles that dot so gracefully the seeming stalk, beneath the seeming flower, can shoot forth a thread, armed, like the fisher’s line, with a barbed hook, which strikes and secures the distant prey; and so the unwary Annelid or Infusory is captured and devoured. In this capacity the creature has been compared to Pope’s spider, who
“Feels at each thread, and lives along the line.” ”

“ Among the Annelids was one with long glassy opalescent bristles surrounding the oral aperture, and projecting forwards to a distance of one and a half [188] inches from the prostomium. Another Annelid (species unknown) was peculiar in having two long barb-like tentacles projecting backwards from the under part of the head. ”

“ Is it not to be marvelled at, that these humble beings should see without eyes, and smell without a nose? It is not affirmed that this is literally and entirely true; but it is exact to a degree enough to prove the wondrous manner in which the sense of touch is made to supersede all the other senses.
"Whether progressing on the solid surface, or moving through water, or tunneling the sand, advancing or retreating in its tube, the Annelid performs muscular feats, distinguished at once for their complexity and harmony. In grace of coil the little Worm excels the Serpent. ”

“ The larvæ of Annelids are very various; but amid their differences there is a recognizable type. “The Trochophore is the typical larval form of the Annelid stem”: a trochophore being a curious spheroidal ciliated structure suggestive of cœlenterate affinities. And this unsegmented larva, representing the remote ancestor from which the many Annelid types diverged, is similar to the larvæ of the Rotifera and the Mollusca: a trochophore is common to all these great classes. ”

“ There is no doubt but that many of our minute annelids are parasitic. Heretofore, however, we have heard little of their ravages. The reason is probably to be found in the fact that so little was known formerly of annelid anatomy that when a worm was discovered its description lacked scientific precision, and so the different parasitic worms remained uncatalogued and unknown. ”

“ Such an origin signifies that the origin of the excretory organs of the vertebrate must not be looked for in the segmental organs of the annelid, but rather in such modified annelid organs as would naturally exist in a primitive arthropod group. The nature of such organs may be inferred, owing to the fortunate circumstance that so primitive an arthropod as Peripatus still exists, and we may conclude that the protostracan ancestor possessed in every segment a pair of appendages and a pair of cœlomic cavities, which extended into the base of these appendages. ”

“ I will only add, under this head, that a fine annelid occurs in the blue agate off Eastbourne; that a “myriapod,” which is among the chromo plates of this volume, has all the characteristics of insect life and motion; and that a spider is the nearest thing I know of, in some respects, to what the “choanite” must have been when that mollusc condensed himself from a cylinder to a sphere. ”

“ Originally the body is composed of a congeries of cells, segment after segment being added, exactly in the same manner as in the case of the Annelida, with which in structure they closely agree, being adapted, however, to live on land. The Annelida seem then to be the ancestors of the Myriapoda, Insects, and Spiders, the Myriapoda retaining much of the Annelid structure through life, whereas the Insect is an Annelid or Myriapod only when in a larval or undeveloped condition. That the development of the different kinds of insects has been gradual. ”

“ The anterior segments of the living annelid only are preserved, since the posterior region is apt to decay and cause the death of the whole. The natives of the Fiji group much relish a form allied to our Lysidice ninetta, and they predict its annual appearance in their seas by observing the phases of the moon. ”

“ Phylum Annelida
Members of the phylum Annelida include the segmented worms such as the common earthworm. Annelids are marine, fresh water, or terrestrial and have apparently been common through much of geologic time. Because of their lack of hard parts, most of these worms have left little direct fossil evidence of their activities in the geologic past. Some annelids secrete straight or coiled calcareous tubes, and fossil worm tubes of this sort (fig. 20) are commonly found attached to brachiopods, mollusks, and other objects. ”

“ The Annelid theory was firmly supported by Eisig, who in his elaborate monograph on the Capitellidæ [410] maintained against Fürbringer the genetic identity of the Annelidan nephridia with the kidney tubules of Vertebrates. The independent discovery by E. Meyer [411] and J. T. Cunningham, [412] of an internal segmental duct in Lanice, into which several nephridia opened, seemed to strengthen this view. ”

“ It is true enough that there is every reason to believe that the arthropod group has been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be looked on as forming one big division, the Appendiculata, all the members of which will some day be arranged in orderly sequence, but the same feeling of certainty does not exist in other cases. ”

“ Sometimes an annelid worm lives inside the shell along with the hermit and often the outside is covered with zoophytes. In some species, as in the British Eupagurus prideauxi, a sea-anemone is constantly found attached to the shell, profiting by the active locomotion of the crab and probably sharing the crumbs of its food, while it affords its host protection by its stinging powers. ”

“ This period extends from the beginning of life up to the time when the annelid was the highest living form yet developed. But in insects and lower vertebrates another system has risen to dominance. This is muscle. The vertebrate no longer devotes all, or the larger part, of its income to digestion and reproduction. ”

“ We have yet to advert to the most interesting class of facts connected with the laws of organic development. It is only in recent times that physiologists have observed that each animal passes, in the course of its germinal history, through a series of changes resembling the permanent forms of the various orders of animals inferior to it in the scale. Thus, for instance, an insect, standing at the head of the articulated animals, is, in the larva state, a true annelid, or worm, the annelida being the lowest in the same class. ”

“ The Annelids, in which the body consists of a long series of connected rings or segments, as in the earth-worm, are one of the highest groups of these worm-like creatures, and some branch of them developed a pair of feet (as in the caterpillar) on each segment of the body and a tough, chitinous coat. Thus arose the early Arthropods, on tough-coated, jointed, articulated animals. Some of these remained in the water, breathing by means of gills, and became the Crustacea. ”

“ But when we come to creatures that are either fixed, or while moving, severally preserve a definite attitude, we no longer find uniformity of surface. The gemmule of a Zoophyte, which during its locomotive stage is distinguishable only into outer and inner tissues, no sooner takes root [88] than its upper end begins to assume a different structure from its lower. The free-swimming embryo of an aquatic annelid, being ovate and not ciliated all over, moves with one end foremost; and its differentiations proceed in conformity with this contrast of circumstances. ”

“ We see this realized in annelid worms known by the name of Heteronereidæ. Certain individuals of small size live on the surface of the water; others, evidently much larger, live at the bottom of the sea and behave quite differently. The eggs and the spermatozoïds proceeding from these two forms differ sensibly from one another, and the difference of form corresponds with that of origin.
We see thus among some of them different males; among others different females: then eggs and spermatozoïds equally different in one and the same animal species. ”

“ A little above the Llandillo rocks, there have been discovered certain convoluted forms, which are now established as annelids, or sea-worms, a tribe of creatures still existing, (nereidina and serpulina,) and which may often be found beneath stones on a sea-beach. One of these, figured by Mr. Murchison, is furnished with feet in vast numbers all along its body, like a centipede. The occurrence of annelids is important, on account of their character and status in the animal kingdom. ”

“ The Annelid theory of Dohrn and Semper was perhaps not so widely accepted as the rival Ascidian theory, but it counted not a few adherents and gave a certain stimulus to comparative morphology. F. M. Balfour, who pointed out about the same time as Semper the analogy between the nephridia of Annelids and the mesonephric tubules of Vertebrates, [405] while not accepting the actual theories of Dohrn and Semper, took up a distinctly favourable attitude to the general idea that Annelids and Vertebrates were descended from a common segmented ancestor. ”

“ Much more likely is it that a fish-like form should have been developed out of a smooth, wriggling, worm-like animal, and it is therefore to the annelids that the upholders of the theory of the reversal of surfaces look for the ancestor of the vertebrate. ”

“ On the shore of our ocean, strangers to these movements, the animated flowers expand their corollæ. Near to the heavy anemone those charming little annelides appear in the sunlight. From a tortuous tube rises a disc, an umbrella, white or lilac, sometimes flesh color. Thrown, itself, a little on one side, it casts off from itself an object which has nothing comparable to it in the whole vegetable world. ”

“ The embryology of the worm has been fully studied, and is replete with marvels. If an egg is examined when the young embryo is almost ready to emerge, it will be possible, through the semitransparent and greatly dilated case, to watch the movements of the worm, trace the current of blood along the elaborate system of vessels, and eventually observe the emergence of the baby Annelid into the world. ”

“ When a new animal is about to be formed, the reproductive segment swells, and after a certain time the infant worm is seen growing from the tail of its parent. When sufficiently developed, the offspring detaches itself, and starts life on its own account. Sometimes before the elder born Annelid is fully formed, the [158] mysterious segment produces a second offspring, and, according to Professor Milne Edwards, as many as six young ones may be generated in succession from the same posterior segment, all of which will for some time continue attached to the parent worm. ”

“ Although deprived of the characteristic setæ of the Annelida, and exhibiting no sectional divisions, they are provided with a sucker-like mouth possessed by Trematoda, but they present no resemblance to them in their reproductive organs. On the other hand, in the arrangement of the nervous system and in their vascular system, the Hirudinidæ resemble Annelida. The head in most of the Annelida 576 is distinctly marked, and furnished with eyes, tentacles, mouth, and teeth, and in some instances with auditory vesicles, containing otolithes. ”

“ Minute Mollusca, both bivalve and univalve, uncouth-formed Crustacea, tiny Starfishes, and especially Annelida, will pretty certainly reward the investigator. The last-named Class occurs in remarkable abundance and variety; while if, after you have gone round the glass, noticing particularly the very edge of the surface-line, you pass your eye, assisted by the lens, carefully over the surfaces of the bits of stone, you will probably find many more creatures, such as tube-dwelling Annelides, the smaller Zoophytes, and several species of the delicate Bryozoa. ”

“ In short, the resemblance in size, position, and colour of every part of the two creatures was so perfect that for a long time I took the corona of the Annelid for a polyp, until, by an accidental blow, I caused all the Myxicolæ of a large coral-stock to shrink suddenly into their tubes, though it was not severe enough to induce an equally rapid movement in the polyps of the apathetic Cladocora." At the time the Professor "felt an almost childish delight at having detected so flagrant an instance of protective mimicry" but soon found reason to doubt this interpretation of the facts. ”

“ Thus the resin which was exuded from trees of long-perished forests is as much a fossil as any portion of the stem, leaves, flowers or fruit, and in some respects is even more valuable to the geologist than more determinable remains of its parent trees, because it has often preserved in admirable perfection the insects which flitted about in the woodlands. The burrows and trails of a worm preserved in sandstone and shale claim recognition as fossils, and indeed are commonly the only indications to be met with of the existence of annelid life among old geological formations. ”

“ The soft bodies of certain species are protected by a coat, or tube, formed by the agglutination of sand, or other foreign substances, as in the Sabella (Lign. 123, fig. 6, p. 385) .
The fossil remains of the testaceous Annelides are very abundant in some deposits; and even the naked, flexible, soft-bodied forms have left proofs of their existence in some of the most ancient sedimentary rocks. Traces of nine species, belonging to five genera of these soft, naked Annelides, have been observed in the Silurian strata of Britain.
Lign. 166. Fossil impression of Nereis. Silurian strata. ”