The generative products in both cases are contained in sacs (Figs. 52, 53, _g_) which lie in the lateral region of the body between the pouches of the alimentary canal. The ova and spermatozoa are conveyed to the exterior by short ducts. Most species are oviparous, though a few viviparous species are known (e.g. _Prosorhochmus claparedii_). CLASSIFICATION.—Nemertines were divided by M. Schultze[141] into:— 1. _Enopla_, in which the proboscis is armed with stylets. 2. _Anopla_, in which the proboscis is unarmed. Although this classification was fairly correct as far as it went, since many other distinctive features were correlated with the presence or absence of armature in the proboscis, still there are several primitive forms belonging to the _Anopla_, which possess characters such as render it necessary to class them together in a separate group. For this reason Hubrecht divided the Nemertinea into three Orders—_Hoplonemertea_, _Schizonemertea_, _Palaeonemertea_; the first of these Orders corresponding with the _Enopla_, and the other two with the _Anopla_. {110}ORDER I. HOPLONEMERTEA. The proboscis is armed. The epidermis rests on a thick layer of connective tissue plentifully supplied with glands, below which is a prominent basement membrane. The muscular layers of the body are two in number, an outer circular and an inner longitudinal. The nerve-trunks lie within the muscular layers of the body and give off regularly-arranged branches. There is no nerve plexus. Each of the cephalic slits generally opens by a pore situated in the centre of a transverse groove, which is beset along one side by a row of shorter grooves at right angles to it. The apparatus consists of a ciliated duct surrounded by nerve tissue, and passing into lobes of tissue which are connected with the brain by thick nerve-cords. The mouth opens rather far forward in front of the brain. The intestinal pouches are symmetrically arranged. Auditory organs are said to exist in some forms, consisting of vesicles containing otoliths. The vascular trunks are connected anteriorly by closed vessels and not by lacunar spaces. [Illustration: FIG. 57.—Anterior end of the everted proboscis (Hoplonemertea). _g.p_, Glandular portion of the proboscis; _l.s_, lateral sacs containing stylets; _m.p_, muscular portion of the proboscis; _s_, stylet; _s.b_, granular basal portion of stylet.] The principal British genera and species[142] are:— _Amphiporus bioculatus_ M‘Int., _A. dissimulans_ Riches, _A. hastatus_ M‘Int., _A. lactifloreus_ M‘Int., _A. pulcher_ Johnst. _Drepanophorus rubrostriatus_ Hubr. (= _A. spectabilis_ Qtrf.). _Tetrastemma ambiguum_ Riches, _T. candidum_ O. F. Müll., _T. dorsale_ Abildg., _T. flavidum_ Ehrenb., _T. immutabile_ Riches, _T. melanocephalum_ Johnst., _T. nigrum_ Riches, _T. robertianae_ M‘Int., _T. vermiculatum_ Qtrf. _Prosorhochmus claparedii_ Keferstein. _Nemertes carcinophila_ Köll., _N. gracilis_ Johnst., _N. neesii_ Oerst.