CRUSTACEA. 



533 



is remarkable for having more than one pair of limbs on the posterior 

 thoracic somites. The abdominal limbs, when present, with the exception 

 of the last pair, never attain the same locomotor importance as do the 

 thoracic. The last somite of the abdomen in Entomostraca carries a pair 

 of furcae anales, variable in character, between which the anus opens. In 

 Malacostraca the furcae and the somite which bears them are modified 

 into an azygos plate or telson with the anus placed on its ventral surface. 



The chitinous cuticle is thin and delicate, or thick, laminate, and 

 calcified. Glands are found in various positions opening on the integu- 

 ment, the limbs, upper lip, round the mouth, where they may have an 

 amylolytic function (many Isopoda] ; or on the under surface of the abdomen 

 (Decapoda), or the lamellae of the brood-pouch (some Isopoda), when the 

 secretion probably serves to attach the ova. In some Cladocera (e. g. Sida) 

 there is a nuchal gland, by means of which the animal can attach itself to 

 some foreign object. A similar gland is present in the young Branchiopod, 

 but is more or less aborted subsequently, and it exists as the dorsal organ 

 in the embryo Arthrostracan. 



The nervous system resembles that of other Arthropoda in its general 

 features. The commissural cords connecting the supra- and infra-oeso- 

 phageal ganglia are sometimes of great length. The first-mentioned 

 ganglion supplies the eyes, the first antennae, and, except in certain 

 Phyllopoda^ the second antennae as well. In the order named the second 

 antennae are innervated (? in all) by a pair of ganglia situated on the 

 oesophageal commissures, and the right and left halves of the post-oral 

 ganglia retain their independence and are united by transverse commissures. 

 In other orders the halves are as a rule united closely ; the ganglia sup- 

 plying the oral appendages are fused into an infra-oesophageal ganglion, 

 and concentration may proceed to such an extent that all the post-oral 

 ganglia become united into a single mass (Decapoda Brackyura} 1 . A 

 stomatogastric system originates in Apus from the first post-oral pair of 

 ganglia ; in the Decapoda from ganglia on the oesophageal commissures 

 which are probably homologous with the first post-oral ganglia of Apus, 

 and additional factors may be derived in the group last mentioned from 

 the supra-oesophageal ganglion 2 . 



1 According to Glaus (Organismus der Phronimiden, Arb. Zool. Inst. Wien, ii. p. 52) the nerves 

 of the second antennae originate from the oesophageal commissures in the Phronimidae, but whether 

 the corresponding nerve-centres are supra- or infra-oesophageal in position is a point he did not 

 determine. In some Isopoda, the right and left halves of the thoracic ganglia, and of the abdominal 

 also when distinct, are .more or less separate, and connected to one another by transverse commissures 

 (Huet, Journal de 1'Anat. et Physiol. xix. p. 305). 



2 A nerve with ganglion cells intercalated from place to place has been observed by Glaus in 

 Phronimidae running on the dorsal aspect of the heart (op. cit. stipra, p. 40). So too in Stomatopoda, 

 in which a ganglion cell corresponds to each pair of ostia (Glaus, ' Circulation of Stpmatopoda? op. 

 cit. v. 1884, p. n). In the same order a ganglion lies just above the epistoma (loc. cit. p. 12). 

 Huet has recently investigated the sympathetic system of Isopoda. He detected an azygos nerve 



