Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/77

the ventral chain, and thus the oesophagus is encircled by a nervous collar. In each segment the ventral ganglia give out branches for the supply of the neighbouring organs and feet. The nerve-cells chiefly occur on the ventral surface and sides of the ganglia. In some (e.g., Tcrebella), while the nerve cords are united in the anterior region, they are separate posteriorly. The eyes are much developed in At dope, certain Syllidse, and others ; but, on the other hand, many are eyeless. Most of the eyes are mere masses of black pigment (sometimes capsulated) situated on the head, destitute of means of accommoda tion, with certain nerve-threads going to them. In Aldope, sclerotic, lens, retina, and other parts exist. In Branchi- omma the eyes occupy the branchiae, and in Fabricia the posterior end of the body. Otolithes are stated to be present in a few (e.g. Arenicola), but on these and other minute points further investigation is required. The sense of touch is extremely delicate in the group ; on the sur face at large, as well as in the tentacles and cirriform processes. Evcirne, Nephthys, Ophiodromus, and others, ex

hibit remarkable irritability.

The Polychasta are for the most part dioecious, the sexual glands being variously developed on the inner sur face of the body-wall or on the dissepiments, and develop ing in the forms of raffs round the vascular axes. The pro ducts are detached into the perivisceral cavity. The ciliated and looped segmental organs of Dr Williams occur very generally, and may conduct outwards the sexual elements, but they have also been supposed to be excrementitious. In Tomopteris their ciliary current carries spermatozoa in wards, and the same may occur in other forms. The structure and deposition of the egg, and the changes in its development, agree with those in other classes. Certain Eunicidse are ovo-viviparous. Occasionally the ova are borne on the back (Polynoe), or in a ventral pouch (Auto- lytus cornutus), until hatched, while in Spirorbis a modifi cation of the operculum acts as an ovisac. The yoxing escape as ciliated organisms with a long whip anteriorly ; but, as development progresses, the cilia are confined to certain zones. A few immature forms, again, such as the young of Nerine, carry remarkable bristles, which are shed before the adult form is reached. Some have arranged the larval forms according to the zones of cilia and the temporary bristles. In the growth of the young animal the development of the new segments takes place between the first and last In certain Nereidae, e.g. Nereis Du- merilii, at least two sexual forms exist, viz., a small adult which develops either ova or spermatozoa in the usual way (except that in the male the elements occur in two testicles placed in one segment), and, secondly, another which becomes transformed into a Heteronereis before the sexual elements are developed. Metschnikoff further found another Nereis which is hermaphrodite. The phenomena of alternate generations is also "observed in Autohjtus, and fissiparity in Filigro.na, the latter, with Spirorbis and A mphiglena, being also hermaphrodite. The female A utolytm quite differs from, the male, and has its body loaded with ova, which pass into a pouch in A. cornutus, Agass., and the products by-aud-by get exit as fiee-swimmmg embryos. The male is wide in front, and has large tentacles, the sperm- sacs occupying the first five bristled segments. From the young of the foregoing is developed so as to complete the seriesthe third kind, viz., the "parent-stock" of A. Agassiz, which differs from both the previous forms. No sexual elements occur in the parent-stock, but the males and females are produced from the body by transverse fission (fig. 8).

Regeneration of mutilated parts is common in the Poly- ch?eta, even to the reproduction of a head.

The tubes formed by many exhibit an amount of pre cision and skill not far removed from the powers of the most ingenious insects. In common with the Nemerteans, their skin exudes a tenacious secretion which coheres under water, and this alone forms the protective tube in some; while in others it is strengthened with mud (fig. 9), sand, gravel, shells, and stones. The secretion varies from the most delicate film in the Syllidse, to the tough parch ment-tubes of CJxetopterus, or the rigid crow-quills of Ilyalincecia (fig. 10). The eyeless Pectinaria, belgica (fig. 11), again, fashions a tube like a straight horn (fig. 12) of minute pebbles or large grains of sand, carefully selected and admirably fixed to each other by a whitish secretion. In placing the grains together in the wall there is no haphazard, but angle fits angle as in a skilfully built wall. Beautiful semi-transparent tubes are constructed by an allied species in the deep sea from the siliceous spicules of sponges. The remark able arborescent or pectinate processes which ornament the free ends of the tubes of certain Terebella; are also noteworthy. The calcareous tubes of the fixed Serpu- (After A. lidae. or the free (e.g. Ditrypa), differ from those of the molluscs in the absence of organic connection between the animal and its protective sheath. Lastly, large coherent masses of coarse gravel and sand-tubes are formed on various beaches by Sabellaria.

FIG. 9. Clymene amphistoma, Sav., with a fragment of tube on the right. (After Savigny.)

The annelids are subject to many para sitic inroads, as by Gregarince, Opalince, and Nematoids, internally; various vege table growths, zoophytes, Loxosomce, and the crustaceans Selius, Selenium, Nereicola, Terebellicola, Sabelliphilus, Chonephilus, and Sabellacheres, externally. Common - salism is likewise not uncommon; thus Folynoe scolopendrina haunts the tubes of Terebdla nebulosa, and it also alternates with Harmothoe tnarphysae in the tunnels of Marpliysa sanguinea; Acholoe astericola frequents the ambulacral rows of Astro- pecten, Malmgrenia castanea, the peri-oral region of Spatangus purpureus, and Nereilepas fucata associates itself with the hermit-crab in Buccinum; while Alcio- pina parasitica lives in the interior of Pleurobrachia, and one of the Amphino- midre in the respiratory cavity of Lcpas.

The Polychseta are all marine, and dis

tributed over the whole surface of the globe, often at very great depths, as early shown by General Sabine, and recently by-the Norwegian dredgings, and FIG. 10. Hya those of the celebrated " Porcupine " and "Challenger" expeditions. Many species are common to the entire North Sea, and extend to tho south as far as Gibraltar, while some northern forms enter the Mediterranean in considerable numbers. The British, species are common to the North Sea, and many range to

the shores of North America. The size of some of the