8 



its epimera and tlie small and rudimentary character of its pleura, in these 

 respects approacliing to the thoracic somites. Its appendages also differ from 

 those of the other abdominal somites in being uniramous. The first pair of 

 abdominal appendages are alike in both sexes, in so far as they consist of a pro- 

 topodite followed by a single branch, probably the endopodite ; but whereas in 

 the female they are slender, in the male they are stout and are deeply grooved 

 along their inner face so as to form, by their apposition, a canal. Their apposition 

 is rendered as intimate as possible by the interlocking of a little roughened 

 facet on the inner edge of each. The canal forms a passage for the sperm, and 

 these appendages in the male are commonly spoken of as copidatory organs. 



The second abdominal somite is distinguished by the large size of its pleura, 

 but is in all respects typical. Its appendages in the female are quite typical ; 

 but in the male they are characterized by the presence of a little lobe on the 

 inner edge of the base of the endopodite, which little lobe can be securely 

 apposed to its fellow of the opposite side, so that both can play in the canal 

 formed by the apposition of the first pair of appendages, and they must there- 

 fore be regarded as accessory organs of copulation. 



The third to the fifth abdominal somites are quite typical, as also are their 

 appendages, which are biramous setose swimming-paddles constructed on the 

 plan already described. In the female these appendages, as also those of the 

 2nd somite, are used at the breeding-season for carrying the fertilized ova and 

 the newly-hatched young. 



The sixth abdominal somite has the pleura less well developed, and the 

 sternum more strongly developed, than those of the other abdominal somites. 

 Its appendages, which are known as the caudal sivhnmerets, are strongly calcified 

 and of large size, and they form with the last abdominal somite the great tail- 

 fan, which is the principal organ of aquatic locomotion. In these appendages the 

 protopodite is short and broad, the exopodite and endopodite are broadly foliaceous, 

 and the exopodite is transversely divided in its distal moiety into two segments. 



The seventh, abdominal somite is the telson, which differs from all the other 

 somites in being a broad thin ^jlate without a^^pendages. On this account, and 

 also because it is destitute of a nerve-ganglion, it is sometimes regarded as 

 merely an outgrowth of the sixth somite. But the facts that the alimentary 

 canal perforates it, and that in many deep-sea species of Decapoda it can 

 apparently be resolved into a somite and a pair of appendages all fused together, 

 are held sufficient to justify its inclusion in the series of somites. 



6. Of the Ees])iratory Organs. 



The organs of respiration of the Crustacea are so intimately connected with 

 the appendages, that they must be treated in immediate sequence to them. 



