ANIMALS WITH JOINTED FEET-THE ARTHROPODS 219 



in the sand and thus fills the statocysts parts of objects that come within its vision, 



through its opening on the dorsal side of The image is thus a mosaic in which the 



the antennule. A bioloo;ist workins; with a sligrhtest movement is readilv detected. This 



marine shrimp, Valeomonetes, allowed the is true, however, only in bright light when 



animal to molt in a clean aquarium, then the pigmented walls of each ommatidium 



supplied iron filings which it promptly are spread over the entire cylinder. In dim 



added to its statocysts. When the animal light the pigment recedes toward the two 



was placed in a magnetic field, it oriented ends of the cylinders, so that the rays of 



itself with respect to the pull of gravity and light pass readily from one ommatidium to 



the pull of the magnet. The special function another. The animal then sees the image 



of the statocysts is their ability to respond superimposed, so that movement is not eas- 



more promptly to the pull of gravity than ily discerned. The image may be more dis- 



would be the case if only the tiny hairs tinct in reduced light, however, than when 



were present. seen as a mosaic pattern (Fig. 11-16). Thus 



The visual mechanism has become very the animal has a means of seeing under 



complex in the arthropods. While arthro- varying light conditions, 



pod eyes vary considerably, a description It is obvious that the Crustacea are well 



for the crayfish eye will convey a general supplied with sense organs that make the 



idea for the entire phylum. The eyes are animals aware of the outside world. How 



located on long movable stalks and can be they interpret the incoming sensations and 



protruded or retracted in order to improve respond to them depends on the develop- 



the vantage point of the observer. The eye ment of the nervous system, 



itself differs markedly from any vertebrate The crayfish and lobster possess a bilobed 



eye (Fig. 11-16). It has an outer rounded brain, relatively large when compared to 



transparent surface called the cornea, that found in lower forms. The brain lies 



which is divided into at least 2,000 tiny between the eyes and is connected with the 



sections, or facets. The facets merely indi- ventral nerve cord by means of a pair of 



cate the outer limits of the units or omma- circumesophageal connectives, very similar 



tidia, which make up the eye of the cray- to the arrangement found in the earthworm 



fish. Each ommatidium is composed of the (Fig. 11-11). There is a large subesopha- 



outer facet, which functions as a lens, a geal ganglion, made up of six fused ganglia, 



series of cells below which make up a sensi- followed by a series of ganglia, one for each 



tive portion, the retinula, and heavily pig- segment. Large nerves extend out from the 



mented regions at the outer and inner ends ganglia to the appendages and to other 



of this cylinder (Fig. 11-16). The sensory parts of the segment. 



cells of each retinula terminate in nerve In addition to its well-developed nervous 

 fibers, which lead directly to the brain, and control the crustacean also possesses hor- 

 the fibers from all the retinulae form a mones, chemical regulators that are pro- 

 nerve equivalent to the optic nerve in ver- duced by one part of the body and affect 

 tebrates. other parts. This has recently been demon- 

 The eye functions as a very efiicient strated in connection with the chromato- 

 organ for photoreception. Light falling phores, located beneath the epidermis, 

 upon tlie lens of a given ommatidium at the which move to the surface and may either 

 proper angle is focused on the sensitive spread out or contract into a tiny mass. The 

 region below and stimulates the sensory alternate expanding and contracting by 

 cells. Since the ommatidia are long cylin- thousands of these bodies have the com- 

 ders, the animal sees tiny points (one for posite effect of changing the color of the 

 each ommatidium stimulated) which are entire animal. In some invertebrates this 



