. A text-book of invertebrate morphology. Invertebrates. 272 INVERTEBRATE MORPHOLOGY. project into the canals of the shell have been stated to be sensory, containing an axial nerve-fibre terminating in a sen- sory cell. The nepliridia (Fig. 120, me) are represented by two or four (Bhynchonella) funnel-shaped short tubes which open by a fimbriated mouth at one extremity into the coelomic cavity and, rapidly narrowing towards the outer end, open by a small pore into the mantle-cavity. In addition to their probable excretory function, these structures, as in some of the Anne- lida, serve also as
. A text-book of invertebrate morphology. Invertebrates. 272 INVERTEBRATE MORPHOLOGY. project into the canals of the shell have been stated to be sensory, containing an axial nerve-fibre terminating in a sen- sory cell. The nepliridia (Fig. 120, me) are represented by two or four (Bhynchonella) funnel-shaped short tubes which open by a fimbriated mouth at one extremity into the coelomic cavity and, rapidly narrowing towards the outer end, open by a small pore into the mantle-cavity. In addition to their probable excretory function, these structures, as in some of the Anne- lida, serve also as ducts for the passage to the exterior of the reproductive elements. These are derived from the coelomic peritoneum and form branching masses (Fig. 120, ov) lying in some cases in the coelomic spaces of the mantle, or in addition extending into the body, as in most Ecardines, or, as in Tere- bratulina, confined to this region. Most of the Brachiopods are bisexual apparently, though it is possible that Lingvla and perhaps some other forms may be hermaphroditic, the male and female elements maturing at different times. Devdopment and Affinities of the Brachiopods.—The Testi- cardines are characterized by the occurrence of a free larval stage destitute of a shell. In Argiope (Fig. 121) it is appar- ently divided into four segments, the most anterior of which bears two eye-spots and assumes an umbrella-like form, long cilia projecting from the margin. The third segment develops two folds which enclose the posterior segment and bear on their margin bunches of setas inserted in seta- sacs and recalling the setae of certain Annelid larvae. After swimming about for a time the larva settles down and fastens itself by the posterior segment and the mantle-lobes turn forward to enclose the anterior seg- ments. The posterior segment becomes the peduncle of the adult, and the shell de- velops on the surface of the mantle-lobes, whose bunches of setae are thrown off. The mouth makes its appe
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