354 Introduction to Animal Morphology. 



a basal protopodite, bearing an endo- and an exo-podite, the 

 latter of which, in the sixth or last post-abdominal segment is 

 transversely divided into two joints. This segment also bears 

 a flat terminal joint or telson, sometimes counted as a seventh 

 somite.* In Astacus it is divided by a transverse suture. 



The heart is short, polygonal, suspended by 3-6 elastic 

 ala cordis. The stomach is suspended by a muscular, partial 

 mesogastrium from the front of the sternum, and from the 

 base of the rostrum. The latter fibres are inserted into the 

 cardiac ossiclef of the gizzard. In Macrura it lies directly 

 beneath the part of the omostegite over the anterior abdo- 

 minal segments, and behind the curved line on that shield 

 which marks the contact of these somites with the posterior 

 thoracic. 



The gills are usually arranged in rows. In Macrura there 

 are three series, the external largest, consisting of the leaf- 

 like gills of the two hinder maxillipedesj and four foremost 

 abdominal legs, the middle and inner of the branched gills 

 springing singly in the second maxillipede, and occasionally 

 from the last abdominal leg, and of pairs of similar gills at- 

 tached to the epimeral plates at the bases of the basipodites 

 of the intervening limbs. These organs are often much 

 divided. There are three sub-orders : 



i. Macrura (Lainille) post-abdomen as long as, or longer 

 than, the cephalothorax, with limbs on all its segments, the 

 last pair and telson expanded into a swimmer. The hinder 

 pair of maxillipedcs do not conceal the others. The embryo, 

 before its maturity, has an exopoditeoneachofits ambulatory 

 legs, like those which remain on the two hinder pair of maxilli- 

 pedes. The omostegite is lengthened laterally into free, 



* It is dorsal, and late in development, never bears limbs, nor a conti- 

 nuously calcified sternum, nor pleurae, and hence is probably not a somite. 



t The gizzard ossicles are pyloric posteriorly, cardiac anteriorly, to the 

 sides of which are attached pterocardiac ossicles, and behind them supero- 

 lateral ossicles join these to the pyloric. 



J On whose outer plicated surface alone gill filaments are developed. 



The presence of this on the hinder maxillipedes has led to the discri- 

 mination of these from the anterior ; thus, in the nomenclature of Spence 

 Bate the two maxillae and first maxillipede are the first, second, and third 



