550 ZOOLOGY SECT. 



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

 referred to (p. 196) : 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 chelae 

 of some male crabs (Gela&imus, Fig. 428, 2) 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 (Palinurus 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 

 Alphem, another Macruran, makes noises by clapping together the 

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

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

 that they 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, the non-motile sperms, 

 and the replacement of the coelome by blood-spaces are funda- 

 mental points of difference from any known Chaetopod. 



As to the mutual relations of the various orders, the Phyllopoda, 

 with their very generalised structure and parapod-like limbs, may 

 be taken as the base of the series. By a differentiation of the 

 post-cephalic limbs, and a reduction in the number of segments, 

 the phyllopod-type easily passes into that of the Phyllocarida. 

 These again lead to the Schizopoda, in which the segments are 

 fixed at the number occurring in all the higher Malacostraca, the 

 caudal styles are no longer present, and the first thoracic legs show 

 an indication of being modified into foot-jaws. From the Schizo- 



