The sexes are separate in _Polygordius_, whilst _Protodrilus_ is hermaphrodite, bearing ova in the first seven segments and testes in the remaining segments. The genital cells are produced from the body-wall in every segment; their mode of discharge is unknown in the male _Polygordius_, though probably the nephridia {245}convey the spermatozoa to the exterior; but in the female the body-wall ruptures to allow the ova to escape, and then the animal dies. The development of _Polygordius_ has been made the subject of very careful study; the larva has long been known, and is a typical "trochosphere" of rather a depressed form. This "trochosphere" larva is of considerable importance, as it makes its appearance in sundry groups of animals in some form or another. Here, in _Polygordius_, it has the appearance of a couple of wide but low cones united together by their bases, which form the equator of the larva. This equator carries a double girdle of cilia, dividing the animal into a preoral and postoral region; for the mouth is placed on one side of the animal between the two girdles, while the anus lies at the apex of the postoral cone, and is surrounded by another girdle of cilia. The alimentary canal is divisible into three regions; it is separated from the body-wall by an extensive space, which contains cells destined to give rise to muscles and nephridia. A nervous system (apical plate) is present at the apex of the preoral cone. This little larva swims freely on the surface of the sea, moving, balancing, and feeding by means of the girdle of cilia. It soon increases in length by the active growth of the apex of the postoral cone, which becomes cylindrical and then segmented externally and internally. The greater part of the original larva remains of the same shape as before, and forms the head (prostomium and peristomium): small tentacles grow out of the preoral lobe, and after a gradual reduction in the relative size of the "head" by the growth of the segmented "body," the animal becomes worm-like and develops into a _Polygordius_.[299] ORDER II. POLYCHAETA. ANATOMY OF NEREIS.—In order to obtain a general idea of a Polychaete worm, it is well to study a concrete example, and for this purpose the common _Nereis_ serves excellently. Several species (see p. 315) occur more or less commonly on our coasts, and the general remarks will apply to one as well as to another. _Nereis pelagica_ Linnaeus reaches a length of 5 to 6 inches, and is about ¼ inch across. It is convex above, nearly flat below. Its {246}colour is brown or bronze. The worm, which is to be found in shallow water, is made up of a considerable number of rings or segments, constituting the "trunk" or "body," terminated at each end by modified segments known as "head" and "tail" (Fig. 122). The segments composing the trunk are all alike, except for small proportional differences, and it will be convenient to describe a "typical segment" before referring to the head or tail.