Text-book of comparative anatomy . observe thicker strands, which form annu-lar commissures between the three principal nerves running inthe longitudinal direction. In the Hoplonemertina the nerve sheath iswanting; the commissures run separately, and sometimes show amarkedly metameric arrangement. In the whole structure of this nervous system a considerable agreement with thatof the Platodcs (Polychidu, Tridada, Trematoda) cannot lie ignored. The brain andthe longitudinal trunks of the Nemertina answer to the brain and the ventral longi-tudinal trunks of the PJatodes. Whether the unpaired dors
Text-book of comparative anatomy . observe thicker strands, which form annu-lar commissures between the three principal nerves running inthe longitudinal direction. In the Hoplonemertina the nerve sheath iswanting; the commissures run separately, and sometimes show amarkedly metameric arrangement. In the whole structure of this nervous system a considerable agreement with thatof the Platodcs (Polychidu, Tridada, Trematoda) cannot lie ignored. The brain andthe longitudinal trunks of the Nemertina answer to the brain and the ventral longi-tudinal trunks of the PJatodes. Whether the unpaired dorse-median longitudinalnerve of the Nemertina answers to the two dorsal longitudinal nerves of the PJafodescannot yet be decided. The longitudinal trunks (and also the brain) lie either IV VERMES—NER VO US S YSTEM 217 imbedded in, or directly under the epithelium (Carinina, Carinella), or they areenclosed in the musculature of the body wall (Cephalothrix, Cerebratuhis), or theylie on the inner side of the musculature (Amphiporus).. Jr* 11LO FIG. HI.—Anterior end of the body of a Nemertian from the side, diagrammatic (partly afterHubrecht). rt, Brain ; iv, proboscis commissure ; fin, medio-dorsal nerve; Iso, cerebral lobe of thelateral organ ; sn, lateral nerve ; mo, mouth ; r, proboscis ; d, intestine; nr, nerve rings, commis-suivs between lateral nerves and the medio-dorsal nerve ; ro, proboscidal aperture. In Malacobdella ganglionic swellings were formerly erroneously described asoccurrinir in the course of the longitudinal trunks. Nemathelminths.—The nervous system of the Xeiiitttoda ( IL) consists of a ring surrounding the oesophagus, the sides of whichoften swell out into a ganglion. A dorsal and a ventral longitudinalnerve arise out of the ring, and these run in the middle line to theposterior end of the body. The two nerves are connected together bytransverse commissures which run under the cuticle outside the muscu-lature. The transverse commissures of the righ
Size: 2223px × 1124px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
