C, Nearly ripe cercariae; cc, cystogenous cells; dr, daughter-redia; dt, limbs of the digestive tract; f, head-papilla; h, eye-spots; h', same degenerating; k', germinal cell; 1, cells of the anterior row; m, embryo in optical section, gastrula stage; n, pharynx of redia; o, digestive sac; oe, oesophagus.
The anterior antennae are fused with the anchoring attachment, whilst the posterior pair is vestigial, and the appendages of the mouth and body present various degrees of degeneration and specialization.
The spinning appendages migrate to the posterior end of the opisthosoma and take up a position close to the anus; the inner branches of the anterior pair either atrophy or are represented homogenetically by a plate, the cribellum, or by an undivided membranous lobe, the colulus.
The nervous system of the embryonic head exhibits three ganglionic masses, anterior to the thoracic ganglionic masses; these three masses subsequently amalgamate and form the sub-oesophageal ganglion, which supplies the trophal segments.
The mouth is divided into two cavities communicating by a narrow orifice, the anterior one containing the incisors and the posterior the molars, the hairy skin of the face being continued inwards behind the incisors.
The opening of the anterior nostrils is prolonged backwards on each side of the face between the nasals and the elongated slender premaxillae.
One set lies in front of the diaphragm, which is the most anterior and complete septum, the rest having disappeared or being much less developed.
The fresh-water Mussels, Anodonta cygnea, Unio pictorum, and Unio margaritiferus belong to the order Eulamellibranchia of Lamellibranch Molluscs, in which the anterior and posterior adductor muscles are equally developed.
The circular space on each side of the basi-temporal (bt.) is the opening of the anterior tympanic recess.
Large doses also depress the nervous system, weakening the anterior horns of grey matter in the spinal cord so as ultimately to cause complete paralysis, and also causing a partial insensibility of the cutaneous nerves of touch and pain.