146 



CLASS CRUSTACEA. 



the Mdaoostraca podopthcdma of Dr. Leach. They are the animals which 

 the ancients particularly noticed under the name of Crustacea, or Crustata. 



Their bodies are usually more or less covered with a kind of shield or 

 shell formed of a single piece, and guarded below with a kind of breast-plate 

 (or plastron), divided transversely into segments, each carrying a pair of 

 true or jaw-like legs. The hinder part of the body, which Latreille calls 

 the post-abdomen, or Urogaster, but which is usually though erroneously 

 called the tail, is only an extension of the end of the alimentary canal, which 

 is defended by seven rings, which are hard above, and soft and membran- 

 aceous below, and carry on their lower part the false legs. 



Latreille divided this Order into two sections according to their general 

 form: 1. Brachyura or Crabs ; and, 2. Macroura or Lobsters. 



ORDER I. BRACHYURA. SHORT-TAILS. 



TAIL short, applied to the chest; destitute of terminal fins, or fin-like ex- 

 pansions; gills solitary; the external aperture of the female organs of 

 generation between the third pair of legs. 



ILLUSTRATIVE EXAMPLE. 



PLATE. 



GRAPSUS. Antenna? four, short, jointed, hid under the hood ; eyes at 

 the angle of the hood, and shortly pedicelled ; body depressed, nearly square ; 

 fins clawed ; the two anterior legs ending in pincers. 



The species of this genus, which are found in the West Indies, being 

 very finely coloured, are called the Painted Crabs. They live principally on 

 putrid animal substances. 



The type of the genus is our first 

 Illustration: Cancer pictus. 



ORDER II. MACROURA. LONQ-TAILS. 



TAIL as long or longer than the thorax, incurved and furnished with two 

 little fins on the sides of the end of the tail ; gills united at their base ; 

 vulva on the first joint of the third pair of feet 



ILLUSTRATIVE EXAMPLE. 



PAGURUS. Antennae four, interior bifid at the apex, peduncle of the 

 interior longer than the setae, which are articulated ; legs dissimilar, formed 

 for walking, anterior didactyle, furnished with a finger and thumb ; tarsi of 

 all conic ; tail soft, furnished with hooks at the tip. 



All the animals of this genus are parasitical, inhabiting the empty cavities 

 of turbinated shells ; they change their habitation with their growth, and 

 are called Soldier-crabs and Hermit-crabs. 



Type of the genus, also our 



Illustration : Pagurus (vel Cancer) Bernardos. 



ORDER III. STOMAPODA. FOOTED-MOUTHS. 



THESE have naked branchiae which adhere to the five pairs of appendages, 

 or fin-feet, attached beneath the abdomen ; the four fore-legs approach close 

 to the mouth, whence the name by which they are designated. 



ILLUSTRATIVE EXAMPLE. 



SQUILLA. This genus belongs to the Umpeltate family of the Stomapodous 

 Order of the Malacostracous Crustaceans of Latreille and Edwards, and is 

 probably one of the most, if not the most, voracious of the class, as indicated 

 by its strong offensive weapons, the claw terminating its prehensile legs 

 being sickle-shaped, and armed with long teeth on one of its pieces, which 

 is received into a corresponding groove on the other. The dorsal shell, of 

 a squarish shape, is divided into three lobes, with a moveable triangular 

 plate on its anterior edge, but posteriorly it does not cover the three pair 

 of ambulatory thoracic legs, which therefore do not seem to belong to it. 

 The branchiae are placed along the lower edge of the body. The b/xly is 

 generally of a more elegant form, and more slender behind the dorsal shell 

 than in the other genera of the same tribe ; the tail is large, and long, 



composed of six segments, the last of which bears some appendages which 

 spread out in a fan-like shape. Their false abdominal legs are in constant 

 motion, and they swim with great speed, impelling themselves by their 

 powerful tail. Lamarck enumerates seven species, of which our 



Illustration : Squilla mantis 



is the type. Edwards names as many as sixteen, which he divides into 

 two sections. 



ORDER IV. ISOPODA. EQUAL-LEGS. 



THESE Crustaceans are known as the Polygonata of Fabricius. They have 

 fourteen well-developed abdominal extremities, none of which are attached 

 to the head but to a distinct segment ; they are hooked at the tip and are 

 destitute of any vesicular appendage at the base ; the five first pairs are 

 lamellar, and subserve the purposes of respiration. 



ILLUSTRATIVE EXAMPLES. 



CYMODOCE. A form of the SpJueromdes, whose generic characters are : 

 Antennae four, small, and unequal, of which the outer two are rather 

 longest ; two sessile eyes ; body oblong, convex, with subimbricated trans- 

 verse sections, and contracting into a ball ; tail with two segments, the last 

 furnished with a pair of natatory scales attached to a single pedicle ; legs 

 fourteen. 



In the Cymodocean Isopods, the leaflets of the fins are projecting and 

 directed backwards ; the sixth segment is not prolonged posteriorly, and in 

 a notch in the last segment there is placed a small blade. 



Illustration : Cymodoce LamarcMi. 



PORCELLUS. Antennae four, joints in the two lateral, which are setaceou s 

 seven, in the intermediate, which are very small two ; tail composed of 

 six segments, and having four appendages, the two external being larger 

 than the two internal ones. They are distinguished from the true Wood- 

 lice (Oniscus), by having seven joints in their antennae, in the latter the 

 number being eight. 



Illustration : ParceUus asellus. 



ORDER V. BRANCHIOPODA. FOOTED-GILLS. 

 THESE Crustacea are for the most part microscopical. Their extremities 

 are lamellar, and membranous ; mouth composed of an upper lip, two 

 mandibles, a tongue and maxillae in some one in some two. Their legs, 

 in general, are fitted for swimming. 



ILLUSTRATIVE EXAMPLES. 



POLYPHEMUS. Body short, globular, inclosed in a bivalve shell ; In MI! 

 divided from the body by a strangulation ; one eye, with two short, inar- 

 ticulated cirris beneath ; antennas large, resembling arms ; tail slender, 

 elevated over the back, and bifurcated ; legs eight. 



Illustration : Polyphemus stagttalis. 



Common in ponds and marshes in Europe. 



DAPHNIA. Shell bivalve ; head apparently armed with two antenna? ; 

 legs eight or ten ; eyes solitary ; tail distinct. The most interesting s]>ecies, 

 and that which is considered the type of the genus, is shown in our 



Illustration : Daphrda pulex (of Muller). 



Found very commonly in ditches and stagnant waters. 



BRANCHIPUS. Antennae setaceous, two or four; eyes two, ]>e<liruhite, 

 compound, moveable ; two moveable horns situated on the front, having a 

 single tooth on the outer side, forked at the apex ; head distinct from the 

 body, which is soft, transparent, elongate, divided into eleven segments ; tail 

 long, sub-cylindrical, articulated, diminishing gradually, and terminated by 

 two ciliated fins ; feet formed for swimming, ciliated, in number rleveii pairs. 



The animals of this genus are very remarkable for many peculiarities in 

 then- form and character. Unlike the Crustacea generally, they have no 

 shell. It appears that they undergo considerable changes of form, in their 

 successive stages of development. They are found in stagnant waters. 



The type of the genus is shown in our 



Illustration : Dranchipus (vel Cancer) stagnate. 



