CRUSTACEA. 



535 



The Podophthalmata possess auditory organs, either open sacs provided 

 with auditory hairs and inclosing foreign bodies as otoliths and placed in 

 the basal joint of the first antenna in the Dccapoda, or closed sacs contain- 

 ing a laminated otolith and situate in the endopodite of the swimmeret as 

 in many Schizopoda. Auditory hairs are said also to occur on other parts 

 of the body in the groups named. 



Mouth and digestive apparatus are wanting in the Rhizocephala 

 among Cirripedia, and the animal is attached to its host by branching 

 processes which spring from its head and penetrate among the viscera. 

 The tract is rudimentary in Proteolepas and many male Cirripedia. In 

 other Crustacea the mouth lies between the mandibles and has an upper 

 lip or epistoma, and a lower lip or metastoma 1 . The latter is often 

 bilobed, and is known in that case as paragnatha. In Euphausia and 

 perhaps in Malacostraca generally the paragnatha are derived from the 

 basal part of the first maxillae. The stomodaeum or oesophageal section 

 of the tract is well developed and often dilated posteriorly. The dila- 

 tation in higher Crustacea is generally provided with chitinoid and some- 

 times calcified plates and teeth, differentiations of the cuticular lining. 

 The mesenteron has no cuticle and corresponds with the archenteron. It 

 varies in length and is rarely convoluted, and is furnished with glandular 

 caeca. These caeca may be short and simple, variable in number, or large 

 branched organs as in Decapoda. The proctodaeum is long in the higher 

 Crustacea, short in the Entomostraca, and is lined by cuticle. The anus is 

 posterior, and may.be dorsal (Copepoda) or ventral (Malacostraca). 



The coelome is filled to a variable degree by the muscles of the 

 somites and limbs and the viscera. It contains a blood-plasma in which 

 amoeboid corpuscles are suspended. The plasma is sometimes tinged red 

 by a colouring matter, either haemoglobin (some Phyllopoda and Cypris) 

 or tetronerythin. In other instances, e.g. Homarus, Squilla (Decapoda), 

 it contains haemocyanin or haemochromogen. A heart may be absent 

 (some Ostracoda and Copepoda, Cirripedia^}. It is placed in the thorax 



nuclei of the retinulae in Amphipoda are situated in the bases of the cells, which are distinctly 

 separated from their outer parts. The nerves to the refractile cells of the Cyclops eye either unite 

 with their internal pointed ends (Grenadier) or with their outer ends (Hartog). 



Patten has examined the eyes of certain Decapoda, (Penaetis, Palaemon, Galathea, Pagurus], 

 especially those of the first-named, in great detail. They have the typical structure given in note 

 p. 452, ante. Pedicels are present in all; and in Galathea the two hypodermis cells corresponding 

 to each corneal facet are modified into a contractile iris. In Branchipus Grubii Patten finds that 

 the retinophorae are grouped in fours ; the corneal cuticula is not facetted ; the corneal hypodermis 

 cells indefinite in arrangement. There are no pedicels, and the outermost set of bacillate retinulae 

 absent. The somatic eyes of Euphausia are held by Sars to be luminous organs (Challenger 

 Reports, xiii. 1885, Schizopoda, pp. 70-72). Patten considers that they have the essential structure 

 of eyes. They possess an extremely thick laminated argentea behind the retina (cf. Mitth. Zool. Stat. 

 Naples, vi. 1886, p. 685 et seqq.). 



1 Unicellular salivary glands are often found in the epistoma, or when that structure is small as 

 in Phronimidae, surrounding the oesophagus. See on Astacus, p. 185. 



