XIII 



DECAPODA 



385 



very long. The relative length of the pairs of arms varies in 

 different species. Two cartilaginous stylets, imbedded in the 

 dorsal mantle, are said by Owen to represent the shell. 



Other genera ; Plnnoctopiis^ body furnished with broad lateral 

 wings which meet at the posterior end ; Cistopus, a large web 

 prolonged along the sides of 

 the arms, fitted with oval 



aquiferous pouches, with pores .^f^^^^^ t 1 ^-^ 

 at their base, between each 

 pair of arms ; Eledone (Fig. 

 244), one row of acetabula; 

 Tritaxeopus^ lapetella. ^1 .>m^.^^ ^^.^^^f^: -^^ 



Sub-order II. — Becapoda. 

 — Body oblong, mouth sur- 

 rounded by four pairs of sessile 

 and one pair of tentacular arms, 

 the latter terminated by a 

 'club'; acetabula pedunculate 



and furnished with a corneous Fig. 244. — J?Zedone^/dro?;anr?? DelleChiaje, 



margin ; mantle margin locked ^^P^^^' ^^'^"^ ^^^'^"'^^ '''^'' " - 



to the base of the funnel by a cartilaginous ai:)paratus ; head 

 and anterior part of body furnished with aquiferous pores ; fins 

 present ; mandibles corneous ; oviduct single, large nidamental 

 glands in the female: shell internal. 



The tentacular arms, which are the princijDal external feature 

 of the Decapoda, are not derived from the same muscular ring as 

 the sessile arms, but arise from the cephalic cartilage, and emerge 

 between the tliird and fourth arm on each side. In Sepia they 

 can be entirely retracted into a kind of pocket behind the eyes, 

 while in Loligo they are simply folded over one another. In 

 Chiroteuthis the arms are six times as long as the body, and the 

 clubs have four rows of denticulate suckers. 



The anterior ventral ^ portion of the mantle is furnished with 

 a singular contrivance for locking it to the funnel, and so render- 

 ing the whole animal more capable of resisting the impact of any 

 force. This contrivance generally consists of a series of ridges 

 or buttons which fit into grooves or button-holes, the ridges being 

 on the interior face of the mantle and the corresponding grooves 



1 It is convenient, but not morphologically correct, to apply the terms ' ventral ' 

 and ' dorsal ' in this sense. 



VOL. Ill 2C 



