CRUSTACEA, 241 



character and disguising the segmentation. The simple 

 annelid eyes are replaced by the compound eyes. 



In the mesodermic organs there are important modi- 

 fications from the annelid type. The simple circular and 

 longitudinal muscles of the body-wall become largely broken 

 up into segmental muscles and limb-muscles. At the same 

 time the perivisceral part of the coelom is replaced by the 

 enormously developed haemocoele or blood-space, the actual 

 body-cavity of an arthropod being a venous blood-space 

 communicating directly with the heart. The paired 

 nephridia or excretory organs are replaced gradually 

 within the sub-phylum by excretory organs of another type. 

 The nephridia are still present in Feripatus, but the coxal 

 glands of Arachnida, and the shell-gland and green-gland of 

 Crustacea, are usually supposed to be much modified 

 nephridial organs. Malpighian tubules appear in Insecta, 

 Arachnida and Mynapoda, Lastly, a centrolecithal type 

 of segmentation appears to be characteristic of the Arthro- 

 poda. 



The Arthropoda have five classes — (i) Crustacea, (2) 

 Protracheata, (3) Myriapoda, (4) Insecta, and (5) Arach- 

 nida. 



Class I. — Crustacea. 



The Crustacea are typically aquatic and breathe by gills. 

 They have two pairs of antennae or feelers on the head. 

 The first five segments are aggregated together into one 

 mass, termed the head, and a number of the other segments 

 may form a thorax and abdomen. The appendages are 

 typically biramous and used for swimming, but more or 

 fewer are modified into legs and jaws. The Crustacea are typi- 

 cally marine and the lower marine types have a free nauplius 

 larva. This larva is pelagic and has a dorsal shield, an 

 unpaired eye and three pairs of swimming appendages 

 round the mouth. The first is uniramous and becomes 

 the antennules; the second and third are biramous and form 

 the aittennce. and mandibles. The nauplius, like the trocho- 

 phore, grows into the adult by elongation of the hind-end of 

 the body and production of fresh segments. In the higher 

 Crustacea, with much yolk in the egg, a stage comparable 

 to the nauplius is passed through in the ^ggr 



M. 1 7 



