VI ARTHROPODA 211 



degenerate blood-vessels — is spoken of as a haemocoelic body-cavity ^ in 

 contradistinction to the coelomic body-cavity such as is present in 

 Annelids. 



There remains to be mentioned a third — less fundamental but still 

 very characteristic — feature of arthropodan structure, namely, that one 

 or more pairs of the appendages in the neighbourhood of the mouth are 

 modified as jaws or other organs connected with the act of feeding. 

 This peculiarity, which no doubt found its commencement in evolution 

 in the fact that these appendages, like other parts of the body, are 

 ensheathed in hard exoskeleton well adapted for crushing the food, is 

 so characteristic that a name -expressing it — e.g. Gnathopoda — would 

 really be preferable to the more commonly used name for the phylum — 

 Arthropoda. 



As will have been gathered, the Arthropods appear to be descended 



Fig. 95. 

 Peripaius (from Sedgwick : Cambridge Natural History), 



from annelid-like ancestors : the primitive form of the body is in con- 

 sequence elongated and worm-like, as may still be seen in Peripatus (Fig. 

 95), or in the Myriapoda, or in the larvae of various insects (Fig. 97, A). 

 In the more highly developed types of arthropod, on the other hand, 

 the body has become more compact and has also become differentiated 

 into distinct regions. Thus in an insect one may recognize distinct 

 head, thorax and abdomen, marked off from one another by more or 

 less distinct constrictions, while in a typical crustacean, such as a 

 Lobster or Crayfish (Fig. 102), one may similarly distinguish cephalo- 

 thorax and abdomen. In the Arachnids there may be three main body- 

 regions distinguishable — the prosoma, mesosoma and metasoma — as in 

 the Scorpions (Fig. 99), or only two — prosoma and opisthosoma— as in 

 the case of Limulus (Fig. 98) or of the spiders. In the Arachnida and 



i From haemocoele — a term sometimes applied to the whole system of 

 spaces in the animal body containing blood, i.e. the cavities of the blood- 

 vessels. 



