602 ZOOLOGY SECT. 



Several instances of commensalism occur in the class. The 

 association of Hermit-crabs with sea- anemones, has already been 



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referred to (p. 208) : another interesting example is the occurrence 

 of the little Pea-crab (Pinnotheres) in the mantle-cavity of Mussels. 

 Other Decapods are found in the intestines of Sea-urchins and 

 Holothurians, and one genus of Crab lives in a cavity in a Coral, 

 the aperture being only just sufficient to allow of a due supply 

 of food and water. 



It is in Crustacea that we find the first indication of characters 

 the purpose of which appears to be their attractiveness to the 

 opposite sex. The immensely enlarged and highly coloured chela3 

 of some male crabs (Gclasimus, Fig. 469, ,9) are said to be used for 

 attracting the female as well as for fighting. The sound-producing 

 organs of some Decapoda have probably also a sexual significance. 

 The Rock-lobster (Palinimis mdgaris) has a soft chitinous pad on 

 the antenna, which it rubs against a projecting keel on the sternal 

 region of the head, producing a peculiar creaking sound ; and 

 Alplieus, another Macruran, makes noises by clapping together the 

 fixed and movable fingers of its large cheloe. The fact that these 

 sounds can be produced at the will of the animals seems to show 

 that the latter undoubtedly possess a sense of hearing, and 

 that the auditory sac is not merely an organ of the sense of 

 direction. 



Affinities and Mutual Relationships.- -That the Crustacea 

 belong to the same general type of organisation as the articu- 

 lated worms is clear enough. The advance in structure is 

 shown in the reduction in number and in the differentiation 

 of the segments, and in the concrescence of those at the 

 anterior end to form a head ; in the hardening of the cuticle 

 into sclerites so as to form a jointed armour; in the jointing 

 and mobility of the limbs ; and in the differentiation of the 

 dorsal vessel into a heart by which the propulsion of the 

 blood is alone performed. The resemblance of the foliaceous 

 limbs of Phyllopods to the parapodia of the higher worms 

 is so striking that one can hardly believe it to be without 

 significance. On the other hand, the absence of transverse 

 muscles and of cilia, and the replacement of the ccelome by 

 blood-spaces, are fundamental points of difference from any known 

 Choetopod. 



As to the mutual relations of the various orders, the Branchio- 

 poda, with their very generalised structure and parapod-like 

 limbs, may be taken as the base of the series. The Ostracoda, 

 Copepoda, and Cirripcdia are best conceived as derivatives, along 



