236 THE CRUSTACEA 



described, but apparently with different functions, occur in many 

 Hyperiidea. In the Phronimidae, where they have been most 

 carefully studied, they occur in all the thoracic legs, and the 

 ducts open at the end of the limb or, in the case of the ante- 

 penultimate pair, on teeth of the palmar edge of the chela. It is 

 supposed that in this case they act as poison -glands. In many 

 Caprellidae groups of gland-cells also supposed to secrete a poison 

 are present in the hand of the second gnathopods, their ducts 

 opening on a prominent tooth of the palmar edge. Finally, glands 

 of similar 'structure may be found, as in many other Crustacea, in 

 the oral appendages and on the oesophageal walls, and are supposed 

 to be salivary in function. 



Nervous System. The ventral nerve-chain presents, in the 

 majority of the Gammaridea, twelve pairs of ganglia connected by 

 double longitudinal commissures, the sub-oesophageal supplying the 

 mouth-parts, followed by seven corresponding to the free thoracic 

 somites and four abdominal ganglia ; but the number may be 

 reduced, especially among the Hyperiidea, by the coalescence of 

 one or two of the anterior thoracic with the sub-oesophageal 

 ganglia, the fusion of the last two thoracic, and the restriction of 

 the abdominal ganglia to three pairs. In the Caprellidea, though 

 four pairs of abdominal ganglia may be distinct in the young, they 

 become in the adult fused into a single mass approximated to the 

 last thoracic ganglion and lying in the penultimate thoracic somite. 

 In Phronima the nerves to the mouth-parts arise from the oeso- 

 phageal commissures close to the under-side of the brain. A post-oral 

 antennal commissure such as exists in the Isopoda is perhaps in- 

 dicated by the presence (in Gammaridea and Caprellidae) of a 

 median foramen piercing the sub-oesophageal ganglionic complex 

 near its anterior margin and giving passage to an unpaired strand 

 of muscle running between the lower surface of the stomach and 

 the lower lip. 



Sense-Organs. The paired eyes of the Gammaridea show great 

 diversity of size and disposition. They are rarely so large as to 

 occupy the greater part of the surface of the head (Trischizostoma), 

 while on the other hand they are in not a few cases quite vestigial 

 or apparently absent. In the Oedicerotidae, as already mentioned, 

 they are coalesced to form a single organ which is advanced to the 

 front of the rostral process, and in a few cases (Tiron, Synopia) a 

 pair of small accessory eyes are placed below the main eyes. This 

 leads to the very remarkable condition found in the Ampeliscidae, 

 which possess two pairs of eyes, each of which is made up of numerous 

 ommatidia differing only in details from those forming the eyes of 

 other Amphipoda, underlying a single lenticular thickening of the 

 cuticle. 



In the Caprellidea the eyes are always small, but in the 



