xi PHYLUM ARTHROPODA 587 



organs of some Decapoda have probably also a sexual significance. 

 The Rock-lobster (PaUnurus vulgaris) 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 

 Alpheus, 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 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 articulated 

 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 coelome by blood- 

 spaces, are fundamental points of difference from any known 

 Chsetopod. 



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 Cirripedia are best conceived as derivatives, along 

 separate lines, of an ancestral form common to them and the 

 Branchiopoda. By a differentiation of the post-cephalic limbs, 

 and a reduction in the number of segments, the branchiopod 

 type easily passes into that of the Phyllocarida, which, though 

 they nearly conform to the malacostracan type of segmentation, 

 have still marked traces of relationship with lower groups in the 

 presence of caudal styles and in their bivalved carapace and 

 foliaceous thoracic appendages. Next to these in ascending order 

 would come the Cumacea with their cephalic carapace coalescent 

 with the first three or four thoracic segments and bounding branchial 

 cavities at the sides of the thorax, but with as more primitive 

 features a biramous character in some of the thoracic appendages 

 and the absence of the fan-like tail-fin. Then a little higher, the 

 Arthrostraca (Tanaidacea, Isopoda and Amphipoda) and the 

 Anaspidacea may be supposed to have branched off from the main 

 trunk at about the same level, and may be regarded, on account 

 of a number of resemblances, as having had a common origin from 

 it. Probably the Anaspidacea are to be looked upon as more 



