XIII. PHYLUM ARTHROPOD A 



A. THE ANATOMY OF THE LOBSTER (OR CRAYFISH) 



For this study either the lobster or the crayfish may be employed, although 

 the former is preferable because of its greater size. The slight differences 

 between the anatomy of these two animals are noted in the course of the out- 

 line (Hegner, chap, xi, pp. 193-225). 



i. External anatomy. Obtain a preserved specimen and place in a dissecting 

 pan. The animal has a hard external covering, the exoskeleton, which cor- 

 responds to the cuticle of the earthworm, and it is a secretion of the ectoderm. 

 It is composed of chitin, rendered hard by the deposit of calcium carbonate in it. 

 Identify anterior and posterior ends, dorsal and ventral surfaces. Is the animal 

 bilaterally symmetrical? Is it segmented? Is it clearly segmented along the 

 whole axis of the body, like the annelids? What part of the body is most evi- 

 dently segmented? what least? 



The body differs greatly from the other invertebrates studied and resembles 

 the frog in that it is divided into definite regions, the head, thorax, and abdomen. 

 Head and thorax are, however, more or less fused into one region, called the 

 cephalothorax. The single piece of the exoskeleton which covers the cephalo- 

 thorax dorsally and laterally is called the carapace. A groove runs across the 

 mid-dorsal region of the carapace and obliquely forward on either side. This 

 is the cervical groove and separates the head in front from the thorax behind. 

 Segmentation has been lost on the dorsal side of the cephalothorax through 

 fusion of segments. 



Another striking difference between the lobster and the other invertebrates 

 previously studied is the presence of jointed appendages. Each segment of the 

 body, is represented by a pair of appendages, and it is thus possible to determine 

 the number of segments even where the lines between them have been lost by 

 fusion. In many arthropods, however, some of the appendages have been lost 

 also, so that the determination of the number of segments in the body is some- 

 times a matter of great difficulty. 



a) The head: The head terminates anteriorly in a spiny pointed projection 

 of the carapace, the rostrum. The head is provided with a number of sense 

 organs, exceeding, in vaiiety and complexity, those of the lower invertebrates. 

 There is a pair of large, stalked, movable eyes, which are probably not appendages. 

 In front of the eyes occurs the first pair of appendages, the antennules, short, 

 forked filamentous outgrowths. Just below these is the second pair of append- 

 ages, the antennae, long, flexible, many-jointed structures. Both antennae and 



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