THE DECAPODA 



305 



Spongieola, Typton, and Eiconaxius live in sponges, many Pontoniinae 

 and Pinnotheridae (as Aristotle knew) in the mantle-cavity of bivalve 

 molluscs ; members of the first-named family inhabit the pharyngeal 

 cavity of Tunicates, and some Pinnotheridae are found in the 

 " respiratory trees " of Holothurians and the rectum of sea-urchins. 

 Many Decapods are constantly found among living corals, and the 

 Hapalocarcinidae live in "galls" on the branches of corals. A 

 very peculiar habit is that of some crabs of the genus Melia, which 

 carry in each cheliped a living Actiniau and use it as a weapon. 



Fio. 182. 



Later stages of Carcimis maeiias. A, young Megalopa. B, older Megalopa. C, post- larval 

 stages, d, dorsal spine of carapace ; r, rostrum (A after Spence Bate ; B and C after Brook. 

 FroufKorschelt and Heider's Embryology.) 



Special interest attaches to the stridulating organs, found in 

 many Decapoda, since their possession is presumptive evidence that 

 the animals^ do have some power of hearing. A few Penaeidae, 

 some Palinuridae, and a considerable number of Brachyura are now 

 known to have stridulating organs in various parts of the body. 

 That of Ocypoda, shown in Fig. 184, is one of the few of which the 

 sound-producing function has been demonstrated by observation of 

 the living animals. It consists of a file-like series of ridges (a) on 

 the inner surface of the propodite of one of the chelipeds, which can 

 be rubbed up and down upon a sharp ridge (b) on the ischiopodite 

 of the same appendage, producing a hissing sound, which probably 



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