152 CRUSTACEA EUCARIDA DECAPODA chap. 



as is kno\vn, however, the direct response of the iiitlividual to 

 the absence of light is limited to the rednction 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 tlie Euphausiidae, make up the 

 Division Eucarida, the memliers 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 (" podoliranchiae ") 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 w^ater 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 



^ Bell, A History of the British Stalk-eyed Crustacea, 1853 ; Heller, Die Grus- 

 taceen ties SiidJichcn Euro2>a, 1863. 



