XII] DECAPODA 217 



appendages. In these two genera we see the beginning of two 

 tendencies which have led the descendants of primitive Schizopoda 

 to differentiate themselves in two different directions. One group 

 have taken to carrying the embryos about until they are fully de- 

 veloped ; at the same time the gills are reduced and the carapace, 

 which is essentially a gill-cover, tends to disappear. This group 

 includes the Stomatopoda, Cumacea and Arthrostraca. 



In the other group the gills and carapace are retained, and 

 though the eggs are for a time carried about attached to the 

 swimmerets the young one passes through a larval stage before 

 becoming adult. This group includes the Decapoda. 



Dr Caiman, one of the greatest authorities on Crustacea, divides 

 the Schizopoda into two divisions, one division including Mysis and 

 its allies, which he classes along with Arthrostraca as Peracarida, 

 and another division including Euphausia and its allies, which he 

 classes with the Decapoda as Eucarida. The Peracarida are dis- 

 tinguished by having oostegites and a liver consisting of few tubules, 

 the Eucarida on the other hand carry the eggs attached to the swim- 

 merets and have a liver of many tubules. There is however much con- 

 venience in grouping together all the low forms as Schizopoda, for 

 which reason the name is retained here, although undoubtedly Mysis 

 shows us the beginning of the type of structure which culminates 

 in the Wood-lice, and Euphausia is not very different in essential 

 structure from the lowest Shrimp. 



Sub-order 2. Decapoda (Eucarida). 



The Decapoda (Gr. Se/ca, ten) derive their name from the circum- 

 stance that the first three pairs of thoracic appendages have become 

 maxillipedes, that is to say have been modified so as to assist in 

 mastication, leaving five pairs of large conspicuous limbs, which 

 have lost all trace of an exopodite, for prehension and locomotion. 



This group includes the Lobsters, Crayfish, Shrimps, Prawns, 

 Hermit-crabs and Crabs, etc. In the division of the Crabs, the 

 Brachyura (Gr. /fyaxv's, short; ovpd, tail), the abdomen is reduced 

 in size and turned up and closely applied to the under surface of the 

 thorax, except when the animal is "in berry" and then the masses 

 of eggs force the abdomen away from the thorax. The last ap- 

 pendages of the abdomen constituting the tail-fin have been totally 

 lost ; of the rest five pairs are retained by the female for the purpose 

 of carrying the eggs, but in the male only the first two pairs, which 

 are used for copulation, are retained. As a rule Crabs are broader 

 than they are long and the breadth is partly due to the large gill 



