412. 



CRUSTACEA. 

 Fig. 413. Fig. 414. Fig. 415 



773 



Fig. 412, Ventral aspect of the cephalo -thoracic portion of the 



Dichelestion. 

 a, trunk or sucker ; b, maxillse. 



Fig. 413, The trunk or sucker magnified, 

 a, thelabrum; b, the mandibles. 



Fig. 414 Sf 415, The maxillae. 



nultimate articulation (the claws, pincers, or 

 cheliferous extremities). 



The extremity occasionally terminates in two 

 articulations presenting no kind of unusual 

 development, but the last of which, termi- 

 nated by a sharp point and armed with teeth 

 or seme, returns upon the preceding one, so 

 as to form a kind of hook or pincer, opening 

 in the opposite direction, (the sub-cheliform 

 extremities of the Squilles and Crevettina;). 

 Lastly, these extremities frequently terminate 

 in a simple acute angle of which the animal 

 can make no use save in locomotion. 



In the Sucking Crustacea, which live parasi- 

 tically on other animals and feed by sucking 

 their blood, the structure of the oral apparatus 

 is extremely different.* Certain pieces which 

 must be considered as analogous to the labium 

 and languette, are elongated, so as to form a 

 trunk or cylindrical tube, of variable length, 

 adapted for sucking, and in the interior of 

 which are lodged the mandibles, now pro- 

 longed so much that they form two slender 

 and pointed processes the extremities of which 

 serve as a lancet. The appendages which in 

 the masticating Crustacea constitute the jaws, 

 here continue rudimentary, and the three pairs 

 of limbs which in the Decapoda complete the 

 oral apparatus, under the name of maxillary 

 extremities, are here transformed into organs 

 of prehension, of different forms, by means 

 of which the parasite attaches itself to its 

 victim. 



In the whole of the Crustacea the intestinal 

 canal presents two openings, the mouth and 



* See our " Recherches sur 1'Organization de 

 la Bouche des Crustaces Suceurs," Ann. des Sc. 

 Nat. t. 28 ; Burmeister's Beschreibung einiger 

 neuen schmarotzer Krebse, in the Acta Acad. 

 Caes. Leop. Nat. Cur. vol. xvii. p. 1. 



the anus, always separated 

 from each other by the whole 

 length of the body. 



The mouth is the mere an- 

 terior and outward expansion 

 of the oesophagus; it is fur- 

 nished with nothing that can 

 properly be compared to a 

 tongue ; the horny and la- 

 mellar organ which writers 

 have sometimes spoken of 

 under this name is nothing 

 more than the lower lip, 

 which has already been de- 

 scribed. 



The oesophagus itself is 

 short ; it rises vertically and 

 runs to terminate directly in 

 the stomach. Its general 

 structure, as well as that of 

 the stomach and whole of 

 the intestinal canal, bears a 

 very close resemblance to 

 what we observe among the 

 superior animals. They each 

 consist of two membranous 

 layers separated by one of 

 muscular fibres, always of 

 greatest thickness in those points in which the 

 most energetic contractions take place, and 

 especially at the entrance into and passage 

 out of the stomach. 



The stomach is of a globular form, and of 

 very great capacity ; it fills a considerable 

 extent of the cephalic cavity, and presents two 

 portions very distinct from one another; the 

 cardiac region, vertically surmounting the 

 mouth and oasophagus, the axis of which is 

 lost in its own ; and the pyloric region, situ- 

 ated behind the former, and forming a right 

 angle with it. 



But the most remarkable feature presented 

 by the stomach of the Crustaceans is the very 

 complex masticatory apparatus it contains. 

 This consists of a considerable number of 

 pieces, the form and disposition of which vary, 

 and are always singularly in harmony with the 

 kind of food taken and the general habits of 

 these animals. The apparatus, as well from 

 the important office it fulfils, as from its 

 being no where else encountered in so perfect 

 a state of development, were worthy of a 

 description which would swell this article to 

 too large a size ; we shall therefore be brief, 

 and merely state generally that it consists of a 

 great number of pieces, so connected as to 

 constitute a kind of solid frame armed in- 

 ternally with tubercles or sharper teeth situated 

 around the pylorus, and capable of being 

 moved so as to bruise or tear in pieces the 

 alimentary matters subjected to their action, 

 and as they are about to pass through this 

 opening.* 



The different pieces composing this appa- 

 ratus vary considerably in the different genera, 



* Vide Cuvier, Lemons d'Anatomie Comparee, 

 t. iv. p. 126, and Milne Edwards, Hist. Nat. des 

 Crustaces, t. i. p. 67, for further details. 



