44 ZOOLOGY 



harm to man, his domestic animals, or his crops; on the 

 contrary, as Darwin has shown, the earthworm acts as nature's 

 ploughman, since it eats its way into the earth, and, after having 

 rilled itself with clay, returns to the surface and voids there 

 the clay which it has brought from the depths. The marine 

 worms form an indispensable food-supply for our edible fish. 



When we leave the creatures usually termed "worms" 

 in order to survey the higher groups of animals, we naturally 

 begin with the phylum of the Arthropoda, since the animals 

 belonging to this group show a good deal of general re- 

 semblance to the Annelida. They show a division of the 

 body into successive rings or somites, and a very similar 

 central nervous system to that possessed by the Annelida, 

 consisting of a brain, collar, and ventral nerve-cord. The 

 great characteristic of the Arthropoda is the existence of a 

 thick external cuticle produced by the cells of the skin. A 

 very thin and delicate cuticle of this kind is found amongst 

 Annelida, but in Arthropoda it becomes so thick and hard as 

 to constitute a veritable armour; and as the animal grows 

 this cuticle is constantly shed and re-formed. Moulting or 

 " ecdysis " occurs much oftener than amongst Nematoda, 

 because the cuticle is not expansible. Encased in such an 

 armour, motion is only possible to the Arthropoda through 

 the existence of folds in the skin where the cuticle remains 

 soft and flexible, and which act as hinges. These flexible 

 spots are termed "arthrodial membranes," and the hard 

 places " sclerites." More or fewer of the somites of the body 

 carry paired outgrowths or "limbs," and the characteristic 

 Arthropod way of moving is not by wriggling or peristalsis 

 like an Annelid, but may best be described as a process of 

 rowing itself along by these limbs if it be an inhabitant of 

 water, or of walking on them if it be a land-dweller. Some 

 of these limbs in the neighbourhood of the mouth are generally 

 modified for crushing the food, and are termed " gnathites " or 

 jaws. One of the most important internal differences between 

 Annelids and Arthropoda is the suppression of the secondary 

 body-cavity or ccelom in Arthropoda and the enlargement of 

 the primary body-cavity into a wide series of spaces. The ccelom 

 is usually large and conspicuous in the embryos of Arthropoda, 

 but as the animal grows it fails to keep pace with the growth 



