PHYLUM ARTHROPODA 1 1 1 



not really the sides of the thorax (which are concealed by the gills)but merely a 

 downward fold from the median cardiac region. 



g) The special sense organs: These are best studied before the animal is 

 dissected. The body and appendages of the lobster bear innumerable hairs, 

 many of which are sensory hairs attached to nerve cells and having tactile 

 functions. Many hairs upon the antennules, antennae, and mouth appendages 

 are also organs of taste and general chemical sense. The head appendages are 

 more sensitive than the others. 



The eyes of the lobster are compound eyes, i.e., they are made up of a large 

 number of simple eyes, or ommatidia, which are radially arranged. The cuticle 

 covering the eye is thin, transparent, and flexible, lacking the calcareous deposit. 

 It is called the cornea. Examine it with the hand lens, noting the minute polyg- 

 onal areas of which it is composed. Each of these is the outer end of one of 

 the ommatidia. Make a longitudinal section through the eye and eye stalk 

 with a sharp knife and examine the cut surface. Observe the numerous black 

 ommatidia radiating from a central white region, which is the optic ganglion. 

 Read Hegner (pp. 205-8) and study Fig. 103. Then remove some of the omma- 

 tidia from the section of the eye, tease on a slide in a drop of water, cover, and 

 examine. You should be able to identify the crystalline cone, the rhabdome 

 surrounded by black pigment cells, and the nerve fiber leading away from each 

 ommatidium. 



The statocyst, or organ of equilibrium, is a thin sac located in the basal segment 

 of the antennules. Remove an antennule and cut off the ventral wall of the basal 

 segment of the antennule. This reveals a thin-walled sac containing sand grains, 

 attached to the dorsal wall of the segment. The assistant should demonstrate 

 the crescent of sensory hairs within the sac (read Hegner, p. 208). 



2. The internal anatomy. With a scissors carefully remove piece by piece 

 the whole dorsal half of the body from rostrum to telson, beginning at the anterior 

 end. In doing so the following points are to be observed; hence read the next 

 section before cutting. Be especially careful not to injure the blood vessels, 

 which are injected with a yellow fluid. 



a) Body wall and muscles: Beneath the hard exoskeleton is a thin membrane, 

 the epidermis, which is the ectoderm and secretes the exoskeleton. Various 

 muscles will be found attached to the shell. In the anterior region under the 

 carapace is the large thin-walled stomach from the anterior and posterior ends 

 of which gastric muscles extend to the carapace. At the sides of the posterior 

 end of the stomach, in front of the cervical grooves, is a heavy fan-shaped mass 

 of muscle. This is the mandibular muscle, which moves the mandible. In 

 removing the dorsal exoskeleton of the abdominal segments notice the longi- 

 tudinal muscles attached to them and trace them forward to their origins on the 

 sides of the thorax. They are the extensors of the abdomen, that is, they 



