. Outlines of zoology. Zoology. CRA YFISH. 255 In c:— r :i fluid and small "otoliths," which seem to be foreign particles. This "ear" seems to be an equilibrating organ, connected with directing the animal's movements some other Crustaceans the audi- tory hairs are lodged in an open depression; this has become an open sac in the crayfish, a closed bag in the crab. Small hairs on the upper lip of the mouth have been said to have a tasting func- tion. The stalked eyes, which used to be regarded as appendages, arise in development from what are called " procephalic l
. Outlines of zoology. Zoology. CRA YFISH. 255 In c:— r :i fluid and small "otoliths," which seem to be foreign particles. This "ear" seems to be an equilibrating organ, connected with directing the animal's movements some other Crustaceans the audi- tory hairs are lodged in an open depression; this has become an open sac in the crayfish, a closed bag in the crab. Small hairs on the upper lip of the mouth have been said to have a tasting func- tion. The stalked eyes, which used to be regarded as appendages, arise in development from what are called " procephalic lobes " on the head. They are compound eyes, that is, they consist of a multitude of elements, each of which is structurally complete in itself. On the outside there is a cuticular cornea, divided into square facets, one for each of the optic elements ; beneath this lie, as in other parts of the body, the nucleated epi- dermal cells. Then follows a focus- sing layer, consisting of many crystalline cones. Each crystalline cone is composed of four crystalline cells, which taper internally, and externally secrete a firm crystalline body. The bases of the crystal- line cones are surrounded by the retinula cells. Each retinula con- sists of five elongated cells arranged about a central axis. Distally, this axis is formed by the crystal- line cone, proximally by a little rod or rhabdome. The rhabdome consists of four little red rods closely apposed together, and connected by a nerve-fibre with the optic ganglion, which lies at the end of the optic nerve. The proximal ends of the retinal cells are. Fig. 106B. —A single eye ele- ment or ommatidium of the lobster.—After G. H. Parker. v., Cornea; , corneal bypo- dermis; cap of crystalline cone; co., crystalline cone and body ', distal retinula elements; , proximal re- tinula elements; R., rhab- dom ; JV., Please note that these images are extracted from scanned page images that may have been digi
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
