126 



ovaries, and (probably) the lateral blood sinuses are 

 remnants of the body cavity. 



The body integument consists of a thin cuticle secreted 

 by the underlying epidermis, beneath which is the dermis 

 containing a plexus of blood capillaries. The next layer 

 is the circular and oblique muscles ; and between that and 

 the botryoidal tissue surrounding the crop, is the more 

 extensive layer of longitudinal muscles. There are also 

 dorso-ventral muscles which pass from the dermis through 

 the circular layer and between the bundles of longitudinal 

 muscles. 



The three central spaces are the crop and two pockets; 

 on the outer side of each pocket is a lateral blood-sinus, 

 and between it and the ventral sinus there is, on each 

 side, a vas deferens. 



PHYLUM ARTHROPODA. 



State the general Characters of Arthropods, and mention the 

 principal Groups. 



The body is segmented and has jointed appendages, in 

 pairs. The segments and appendages are variously modified 

 according to use ; one or more pairs of the appendages 

 become gnathites or jaws, and these show striking modifi- 

 cations according to the nature of the food. The cuticle, 

 stiffened by the deposit of a waste substance (chitin), 

 forms a tough exoskeleton, often rigid except about the 

 joints ; and it is periodically moulted (ecdysis). Ciliated 

 epithelium is generally absent. The coelome is greatly 

 reduced ; there is instead a haemocoele (of blood-carrying 

 spaces). The heart is dorsal. There is a double ventral 

 nerve cord with ganglia, and a nerve ring (round the gullet) 

 with cerebral ganglia. The sexes are usually separate. 

 The eggs generally contain a great quantity of yolk. The 

 development frequently shows metamorphosis. The Ar- 

 thropods show great variation in habits, and there is a 

 remarkable diversity of forms. 



NOTE. For the principal groups of Arthropods, see 

 lt Catechism" Zoology, Part /., page 18. 



