Methodological guidelines on tasks
- These directions may be used for the crayfish without much modification. The smaller size of these animals will make it more difficult to trace some of the nerves and blood-vessels hair-like spines along its free border. What purpose do these serve ?
- The abdomen is composed of seven movable segments, each bearing a pair of jointed appendages except the last, which is sometimes not considered a true segment and is called the telson. Each abdominal segment consists of a dorsal piece, the tergum, which is continued as a free plate laterally (the pleuron), and of a ventral piece, the sternum. Move the abdominal segments and see where they are hinged. How are the terga and sterna arranged to allow free movement? In the thorax the sterna, though fused, can be distinguished. There are eight segments in the thorax.
- Appendages. Aside from the stalked eyes, whose homology with true appendages is doubtful, there are nineteen pairs. These are, counting from before backward: antennules, antenna, six pairs of mouth appendages, five pairs of walking legs (pereiopods), of which the first are the claws or chelce, and six pairs of swimmerets (pleopods}. In the male, the first two pairs of pleopods are modified to form copulatory organs.
(a) Turn one of the fifth pair of pleopods forward and examine its posterior aspect. It consists of a basal piece, the protopod, a lateral branch, the exopod, and a median branch, the endopod. This branched type of appendage is designated as biramous. What is its use? Compare with this the modified sixth pair of pleopods, called the uropods.
Make a drawing of one of the fifth pleopods.
(b) In front of the chelae will be seen the sixth pair of mouth appendages, the third maxillipeds. Remove that of the right side and compare it with the fifth pleopod. In addition to the protopod, exopod, and endopod, it bears a long blade, the epipod, which extended into the gill chamber. The protopod is com- posed of two segments, coxopod and basipod; the endopod of five segments, ischipod, meropod, carpopod, propod, and dactylopod. The exopod is composed of one long and many short segments. How is the appendage modified to serve in feeding?
Make a drawing of the third maxilliped.
(c) Remove the remaining five mouth appendages and compare each with the third maxilliped. These are, beginning posteriorly, the second maxilliped, first maxilliped, second maxilla (with a broad paddle, the scaphognathite, the use of which should be understood), first maxilla, and the mandible. Just back of the mandibles are two small flaps, the paragnatha, which are not true appendages. Do you understand the use of each of these appendages? Most of the appendages have parts that may be compared with the typical biramous appendage, but they are much modified to serve special functions, and the exact homologies are not important. Between the mandibles note the mouth, bounded in front by the labrum.
Drawings of these appendages may be made if time permits.
(d) The antenna are biramous. Notice on the ventral side of the basal joint of an antenna the opening of the green gland or nephridium.
(e) The antennules, though branched, are not considered to be of the biramous type. Do you know why ? Remove one and note on the dorsal surface of the basal joint a groove at whose median extremity is a small hole, the opening into the otocyst. Do you know the probable function of the antennules and of the otocysts? What reason is there for having both antennules and antennaB?
(f) Compare the pereiopods with the third maxilliped. Which is lacking, endopod or exopod? Examine each of the joints of one of these appendages and see in what directions the appendage may be moved. Are there any ball-and-socket joints? Compare the chelae with the other pereiopods and see how they differ. To what part of a chela does the last segment of the last pereiopod correspond? What reason is there for having these appendages different? Do you think the arrangement of the appendages would aid the lobster in climbing over rough bottom?