Outlines of zoology . glia of theventral chain, nerves go to thehind-gut. If the brain be regard-ed as the fusion of two pairs ofganglia, as the development sug-gests, and the sub-oesophageal ascomposed of six fused pairs, therthese, along with the eleven otherpairs of the ventral chain, givea total of nineteen nerve-centres,—a pair for each pair of append-ages. Sensory system.—A skinclothed with chitin is not Fig. 143likely to be in itself verysensitive, but some of thesetae are, and some ob-servers describe a peri-pheral plexus of nerve-cellsbeneath the sets are not mereoutgrow
Outlines of zoology . glia of theventral chain, nerves go to thehind-gut. If the brain be regard-ed as the fusion of two pairs ofganglia, as the development sug-gests, and the sub-oesophageal ascomposed of six fused pairs, therthese, along with the eleven otherpairs of the ventral chain, givea total of nineteen nerve-centres,—a pair for each pair of append-ages. Sensory system.—A skinclothed with chitin is not Fig. 143likely to be in itself verysensitive, but some of thesetae are, and some ob-servers describe a peri-pheral plexus of nerve-cellsbeneath the sets are not mereoutgrowths of the cuticle, but are continuous with theliving e{)idermis beneath; and though some are only fringes,both experiment and histological examination show thatothers are tactile. On the under surface of the outer fork of the antennulesthere are special innervated setae, which have a smellingfunction. Other specialised setae have sunk into a sac at the baseof the antennules, and are spoken of as auditory. The sac. Section of compound eyeof My sis vulgaris.—After Gren-acher. ., Muscle of eye-stalk; 1-4 ganglionicswellings in the course of the opticnerve ; «., the nerve fibrils passing upto the retinulae; rA., the rhabdoms;re.^ elements of retinula:; ;*., band ofpigment; c, crystalline cones; Cd>., thecorneal facets with the subjacent nuclei. 288 PHYLUM ARTHROPODA. opens by a bristle-guarded slit on the inner upper corner ofthe expanded basal joint, and contains a gelatinous fluidand small otoliths, which appear to be foreign ear seems to be an equilibrating organ, connectedwith directing the animals movements. In some otherCrustaceans the auditory hairs are lodged in an open de-pression ; this has become an open sac in the crayfish, aclosed bag in the crab. Small setae on the upper lip ofthe mouth have been said to have a tasting function. The stalked eyes, which used to be regarded as append-ages, arise in development from what are called procephaliclo
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
