xii CRUSTACEA 157 



appear jointed, but the visible rings do not mark true joints. 

 In the structure of its heart, Peripatus resembles an Arthropod, 

 and it breathes by tracheae, as do all the air -breathing 

 Arthropods ; and yet, on the other hand, there is within the 

 body a series of paired excretory organs recalling those of 

 Annelids, and this resemblance becomes much more striking 

 when we trace their development, for it then becomes obvious 

 that they represent all that remains of a segmented coelom 

 which is clearly present in the embryo Peripatus, as it is in 

 the adult worm. The continuous body-cavity which is con- 

 spicuous in the adult is developed from the blood-vascular 

 system. 



Peripatus may therefore be looked upon as a real link 

 between the air-breathing Arthropoda and the Annelida, 

 though its connection seems to be rather with the Myriapods 

 (p. 201) and the Insects than with the Arachnids, which 

 probably had a different origin. 



Class I. : CRUSTACEA 

 (CRABS, LOBSTERS, SHRIMPS, WATER FLEAS, ETC.) 



General Crustacea are aquatic Arthropods breathing 

 Character- typically by gills. These gills are thin-walled 

 istics. tubular processes from the body, through which 

 the blood circulates and is thus brought into close contact with 

 the surrounding water and its dissolved oxygen, the necessary 

 interchange of gases for purifying the blood taking place by 

 diffusion through the delicate membrane covering the gills. 

 In some of the simpler forms only, gills are absent, the inter- 

 change of gases taking place over the general body surface. 



The limbs of Crustacea, with the exception of the antennae, 

 are typically two-branched, a characteristic peculiar to them 

 amongst Arthropods. In the adult of some forms some un- 

 branched limbs may be found, but these have arisen as a modi- 

 fication of the two-branched structures found in the young. 



Hi her ^ n ^ e hig ner Crustaceans, a definite and constant 



Crustacea number of segments and appendages occurs, namely, 



(Mala- twenty segments and nineteen pairs of appendages; 



ostraca). ^ j agfc tail-segment alone bearing no appendages. 



In these higher forms, however, the segments of the head and 



