152 CRUSTACEA—EUCARIDA—DECAPODA CHAP. 
as is known, however, the direct response of the individual to 
the absence of light is limited to the reduction or disappearance 
of the pigment, and does not extend to those structural changes 
in the ommatidia which are characteristic of so many deep-sea 
forms. 
Order II. Decapoda.’ 
The Decapoda, together with the Euphausiidae, make up the 
Division Euearida, the members of which differ from the Orders 
hitherto described in a number of characters, e.g. the presence of a 
carapace covering the whole of the thorax, the absence of a brood- 
pouch formed of oostegites, the presence of a short heart, of sper- 
matozoa with radiating pseudopodia, and of a complicated larval 
metamorphosis, of which the Zoaea stages are most prominent. 
The Decapoda differ from the Euphausiidae chiefly in the 
anterior three thoracic limbs being turned forwards towards the 
mouth to act as maxillipedes, and in the five succeeding thoracic 
limbs being nearly always uniramous and ambulatory or chelate ; 
there are typically present three serial rows of gills attached 
to the thoracic segments, an upper series (“ pleurobranchiae ”) 
attached to the body-wall above the articulation of the limbs, 
a middle series (“arthrobranchiae”) attached at the articulation 
of the limbs, and a lower series (“ podobranchiae ”) attached to 
the basal joints of the limbs. These gills are enclosed in 
a special branchial chamber on each side of the thorax, formed 
by lateral wings of the carapace known as “ branchiostegites.” 
The gills of each series are never all present in the same animal, 
the anterior and posterior members showing a special tendency 
to be reduced and to disappear. In this manner “ branchial 
formulae” can be constructed for the various kinds of Decapods, 
which differ from the ideal formula in a manner distinctive 
of each kind. The second maxilla is always provided with an 
oar-like appendage on its outer margin (exopodite), known as 
the “scaphognathite,’ which, by its rhythmical movement, 
keeps up a constant current of water through the gill-chamber. 
A complicated auditory organ is present on the basal joint of 
the first antennae; this is a sac communicating with the 
exterior and lined internally with sensory hairs. The animal is 
1 Bell, 4A History of the British Stalk-eyed Crustacea, 1853 ; Heller, Die Crus- 
taceen des Siidlichen Europa, 1863. 
